Add /graph G0: serial graph engineering mode over the goal engine

A deterministic DAG scheduler layered on the existing goal engine: /graph
<objective> decomposes the objective via a graph-planner subagent, gates the
result through Agentproof-style static validation (cycles, unknown deps,
duplicate slugs, caps), appends a structural final-verification node, then
executes each node as one ordinary goal — planner, worker loop, adversarial
verifier, budget and pause machinery all reused verbatim. The in-turn loop
advances nodes within the same turn (multi-loop closed loop); goal-side
auto-pauses cascade to the graph at a single chokepoint; node goals are armed
with the remaining graph budget so mid-node overruns trip graph-wide.

Gated by KIGI_GRAPH=1 (default off) + the goal harness. State persists to
<session_dir>/graph/state.json with a clear-tombstone; per-version immutable
baselines and per-node artifact archives live under graph/<graph_id>/.
Restore demotes Active->UserPaused and Running->Ready (verifier-gated re-run).

Review hardening (adversarial multi-agent pass, 24 confirmed findings fixed):
the goal-inactive loop break now consults the graph seam (mid-turn
classifier-disabled completions can no longer strand an Active graph), the
pause cascade fires even when no node goal is in flight (cancel during
planning), node bookkeeping precedes the long setup await, /graph clear only
resets the engine it owns, budget trips terminally fail the in-flight node,
and the pause transitions in /goal pause + /graph pause no longer hide inside
debug_assert! (a release-build no-op inherited from upstream).

Tests: 4908 kigi-shell lib tests green, including a serial 4-node closed-loop
e2e, restore/resume, cascade, mutual-exclusion, planning-retry, persistence
round-trip + tombstone, and status rendering.

crates/codegen/kigi-shell/src/agent/subagent/handle_request.rs

@@ -1036,6 +1036,7 @@ pub(crate) async fn handle_subagent_request(
             None,
             None,
             None,
+            None,
             if verbatim_mirror_fork {
                 None
             } else if let Some(scope) = agent_memory_scope {
@@ -1069,6 +1070,8 @@ pub(crate) async fn handle_subagent_request(
             ctx.app_builder_deployer_config.clone(),
             ctx.write_file_enabled,
             ctx.goal_enabled,
+            // Graph mode is a depth-0 harness; child sessions never drive it.
+            false,
             true,
             ctx.ask_user_question_enabled,
             ctx.client_hooks.clone(),