Add /graph G6: plan-boundary topology optimizer

A restricted optimizer pass now reviews the graph at plan boundaries —
right after initial planning and piggybacked on each replan version
boundary, never mid-execution. An optimizer subagent may emit four ops
over Waiting/Ready nodes only: remove_dep (delete a false dependency,
restoring parallelism — the highest-value edit), reorder (pending
priority for the serial scheduler), merge (fold two tiny nodes; specs
concatenate, deps union, dependents re-point, absorbed self-deps drop),
and split (2-3 focused replacements inheriting the original's deps and
dependents). The optimizer changes graph DATA only; the executor stays
pure deterministic Rust. KIGI_GRAPH_OPTIMIZER=0 disables it entirely.

apply_optimization enforces the contract twice: per-op checks
(pending-only targets, known ids, terminal node untouchable, dead-node
deps rejected as DeadDep, merge/split targets with non-pending
dependents rejected with the true reason instead of tripping the
immutable invariant later), then FINAL invariants — every non-pending
node byte-identical in the result, the gn-final gate rebuilt over all
survivors, node cap, whole-graph acyclicity, and a BIDIRECTIONAL status
re-derivation for pending nodes (adversarial review caught the critical
hole: a merge grafting unsatisfied deps onto a Ready node would
otherwise dispatch it ahead of its new prerequisites, since
recompute_ready is promote-only). Applied passes bump plan_version,
freeze an immutable baseline, and consume a slot of the SHARED replan
cap; an explicit {"ops": []} is a respected free no-op; any failure
degrades to keeping the current plan. Plumbing reuses a new shared
artifact-pass runner (stale-artifact delete, size cap, missing-file
fail-closed) extracted from the replanner.

Tests: remove_dep parallelism restore + loud no-such-dep, immutable and
terminal-node rejections across all four ops, merge/split dependent
rewiring incl. final-gate rebuild and intra-split dep resolution,
result-cycle rejection, dead-dep splits, Ready-demote-on-merge, and
three e2e flows — false-dep removal proven ACTUALLY parallel by the
held-reply fan-out gate, OPTIMIZER=0 spawning zero passes, and the
shared-cap guard. kigi-shell 4959 lib tests green; clippy clean.

crates/codegen/kigi-shell/src/session/acp_session_impl/graph.rs

@@ -388,6 +388,8 @@ impl SessionActor {
         self.graph_tracker.lock().install_nodes(nodes);
         self.persist_graph_state();
         tracing::info!(total, "graph: DAG installed, launching first node");
+        // Plan-boundary optimizer pass ① (post-initial-planning).
+        self.maybe_optimize_graph().await;
 
         match self.drive_graph().await {
             Some(reminder) => GraphSetupOutcome::Inference {

crates/codegen/kigi-shell/src/session/acp_session_impl/graph_optimize.rs

@@ -0,0 +1,171 @@
+//! Topology optimizer (G6): a plan-boundary review pass that may issue
+//! a RESTRICTED set of graph edits — remove false deps (restoring
+//! parallelism), reorder pending priority, merge tiny nodes, split
+//! oversized ones — over Waiting/Ready nodes only.
+//!
+//! The optimizer changes GRAPH DATA only; the executor stays pure
+//! deterministic Rust. It fires ① right after initial planning and
+//! ② at each replan boundary (piggybacked), never mid-execution.
+//! Applied (non-empty) passes bump `plan_version`, freeze a baseline,
+//! and consume a slot of the SHARED replan cap; an explicit `[]` is a
+//! respected no-op consuming nothing. Failure degrades — the current
+//! graph keeps running. `KIGI_GRAPH_OPTIMIZER=0` disables entirely.
+
+use std::sync::Arc;
+
+use super::super::goal_planner::{ChannelSpawner, GoalPlannerSpawner};
+use super::super::graph_plan;
+use super::super::graph_planner::{ArtifactPassSpec, run_graph_artifact_pass};
+use super::SessionActor;
+
+const OPTIMIZER_PROMPT_TEMPLATE: &str = include_str!("../templates/graph_optimizer_prompt.md");
+
+impl SessionActor {
+    /// One optimizer pass at a plan boundary. No-op when disabled, when
+    /// the shared cap is exhausted, or when the graph is not Active.
+    pub(super) async fn maybe_optimize_graph(&self) {
+        if !self.graph_optimizer_enabled {
+            return;
+        }
+        let (replan_runs, current_graph, history_text, graph_file, next_version) = {
+            let tracker = self.graph_tracker.lock();
+            let Some(state) = tracker.snapshot() else {
+                return;
+            };
+            if state.status != crate::session::goal_tracker::GoalStatus::Active {
+                return;
+            }
+            let compact: Vec<serde_json::Value> = state
+                .nodes
+                .iter()
+                .map(|n| {
+                    serde_json::json!({
+                        "id": n.id,
+                        "title": n.title,
+                        "spec": n.spec,
+                        "status": format!("{:?}", n.status),
+                        "deps": n.deps.iter().map(|d| d.on.clone()).collect::<Vec<_>>(),
+                    })
+                })
+                .collect();
+            let history_text = state
+                .history
+                .iter()
+                .rev()
+                .take(12)
+                .map(|h| {
+                    format!(
+                        "- {:?} {} {}",
+                        h.event,
+                        h.node_id.as_deref().unwrap_or("-"),
+                        h.detail.as_deref().unwrap_or("")
+                    )
+                })
+                .collect::<Vec<_>>()
+                .join("\n");
+            (
+                state.replan_runs,
+                serde_json::to_string(&compact).unwrap_or_default(),
+                history_text,
+                tracker
+                    .artifacts_dir()
+                    .join(format!("optimize.v{}.json", state.plan_version + 1)),
+                state.plan_version + 1,
+            )
+        };
+        if self.graph_replan_cap == 0 || replan_runs >= self.graph_replan_cap {
+            tracing::info!(
+                replan_runs,
+                cap = self.graph_replan_cap,
+                "graph optimizer: shared cap exhausted; skipping pass"
+            );
+            return;
+        }
+        let Some(event_tx) = self.tool_context.subagent_event_tx.clone() else {
+            return;
+        };
+        let objective = self
+            .graph_tracker
+            .lock()
+            .objective()
+            .map(str::to_owned)
+            .unwrap_or_default();
+        let parent_prompt_id = self
+            .current_prompt_id
+            .lock()
+            .expect("current_prompt_id mutex poisoned")
+            .clone();
+        let spawner: Arc<dyn GoalPlannerSpawner> = Arc::new(ChannelSpawner {
+            event_tx,
+            parent_session_id: self.session_id_string(),
+            parent_prompt_id,
+            cwd: Some(self.tool_context.cwd.as_str().to_owned()),
+            role_override: Default::default(),
+            events: Some(self.events.writer()),
+        });
+        let tool_names = self.resolve_inherit_role_tool_names().await;
+        let sections = format!(
+            "OBJECTIVE:\n{objective}\n\nCURRENT GRAPH:\n{current_graph}\n\n\
+             EXECUTION HISTORY:\n{history_text}\n"
+        );
+        tracing::info!(next_version, "graph optimizer: firing");
+        let json = match run_graph_artifact_pass(
+            spawner,
+            ArtifactPassSpec {
+                template: OPTIMIZER_PROMPT_TEMPLATE,
+                sections: &sections,
+                graph_file: &graph_file,
+                tool_names: &tool_names,
+                role: "graph optimizer",
+            },
+        )
+        .await
+        {
+            Ok(json) => json,
+            Err(reason) => {
+                // Degrade: an enhancement pass never blocks the graph.
+                tracing::warn!(%reason, "graph optimizer: pass failed; keeping current plan");
+                return;
+            }
+        };
+        let existing = self
+            .graph_tracker
+            .lock()
+            .snapshot()
+            .map(|s| s.nodes.clone())
+            .unwrap_or_default();
+        match graph_plan::apply_optimization(&existing, &json) {
+            Ok(None) => {
+                tracing::info!("graph optimizer: no ops (already good)");
+            }
+            Ok(Some(optimized)) => {
+                let n_before = existing.len();
+                let n_after = optimized.len();
+                self.graph_tracker.lock().install_optimized_nodes(optimized);
+                let all_nodes = self
+                    .graph_tracker
+                    .lock()
+                    .snapshot()
+                    .map(|s| s.nodes.clone())
+                    .unwrap_or_default();
+                if let Err(err) = self.write_graph_baseline(&all_nodes).await {
+                    tracing::warn!(%err, "graph optimizer: baseline write failed (audit gap only)");
+                }
+                self.persist_graph_state();
+                tracing::info!(
+                    next_version,
+                    n_before,
+                    n_after,
+                    "graph optimizer: plan optimized"
+                );
+                self.send_slash_command_output(&format!(
+                    "Graph optimized (v{next_version}): {n_before} → {n_after} node(s)."
+                ))
+                .await;
+            }
+            Err(err) => {
+                tracing::warn!(%err, "graph optimizer: ops rejected; keeping current plan");
+            }
+        }
+    }
+}

crates/codegen/kigi-shell/src/session/acp_session_impl/graph_replan.rs

@@ -198,6 +198,9 @@ impl SessionActor {
                          discovered work."
                     ))
                     .await;
+                    // Plan-boundary optimizer pass ② (piggybacked on the
+                    // replan version boundary).
+                    self.maybe_optimize_graph().await;
                     return;
                 }
                 GraphPlannerOutcome::Invalid { reason } if attempt == 1 => {

crates/codegen/kigi-shell/src/session/acp_session_impl/spawn.rs

@@ -1104,6 +1104,7 @@ pub(crate) async fn spawn_session_actor(
         graph_concurrency: effective_config.resolve_graph_concurrency(),
         graph_node_rounds: effective_config.resolve_graph_node_rounds(),
         graph_replan_cap: effective_config.resolve_graph_replan_cap(),
+        graph_optimizer_enabled: effective_config.resolve_graph_optimizer_enabled(),
         graph_project_dir,
         graph_project_lock: std::cell::RefCell::new(None),
         goal_turn_task_ids: parking_lot::Mutex::new(std::collections::HashSet::new()),