Add /graph G1: parallel fan-out with worktree isolation and merge-back

With KIGI_GRAPH_CONCURRENCY > 1 (default 3, clamp [1,8]) and >=2 Ready nodes,
drive_graph — the single dispatch loop shared by setup/advance/resume — runs
parallel batches: each node executes as a bounded worker<->verifier subagent
loop (KIGI_GRAPH_NODE_ROUNDS, default 3; general-purpose children with the
full implementer toolset; worktree isolation on round 1, resume keeps context
and worktree on later rounds; NODE_RESULT/NODE_VERDICT terminal contracts
parsed fail-closed with fence-stripping and line anchoring). Achieved nodes
merge back SEQUENTIALLY via kigi_workspace apply_worktree in Merge mode; a
conflict fails the node, block_dependents fires, and surviving chains keep
going. gn-final always runs serially on the full goal engine. Concurrency=1
is byte-identical to the serial G0 path; non-git projects degrade to serial.

Merge primitive hardened for real use (kigi-workspace): the 3-way apply is
now byte-safe (binary files no longer read as UTF-8 and silently deleted)
and gains the identical-content rule (ours==theirs => already present, not a
conflict) — without it, any dirty file inherited via PreserveWorkingTree
false-conflicted every node merge and multi-wave graphs self-poisoned.

Adversarial review pass (16 confirmed findings, all fixed): in-session
resume demotes orphaned Running nodes instead of wedge-pausing forever after
a mid-batch Esc; cancel re-sweeps subagents AFTER the turn abort so workers
spawned in the cancel window die too; empty child ids are never adopted as
resume targets (no unisolated escape to the shared tree); a successful
isolated round returning no worktree fails the node (soft-fallback can no
longer put N writers in one tree); main-HEAD movement during a batch aborts
merges instead of reverse-applying external commits; failed nodes still
charge the token budget; budget trips terminally fail the in-flight node;
runaway (>600s) rounds are cancelled by spawn id and retried via resume;
worker summaries are marker-sanitized before verifier embedding. Merged
worktrees are removed immediately (storage discipline); failed nodes keep
theirs for postmortem.

Tests: 4922 kigi-shell lib tests green (58 graph-specific), including
fan-out proven by a held-reply gate, real-git batch merge with cleanup
assertions, budget charging across verdicts, cap trimming, resume-after-
cancelled-batch, and backgrounded-round cancel semantics.

crates/codegen/kigi-shell/src/session/graph_planner.rs

@@ -193,10 +193,12 @@ mod tests {
         ]))
     }
 
-    fn tmp_graph_file(name: &str) -> PathBuf {
-        let dir = std::env::temp_dir().join(format!("kigi-graph-planner-test-{name}"));
-        std::fs::create_dir_all(&dir).unwrap();
-        dir.join("graph.json")
+    /// Self-cleaning temp home per test: never leak dirs into the OS
+    /// temp root (storage discipline — see AGENTS.md gates).
+    fn tmp_graph_file(_name: &str) -> (tempfile::TempDir, PathBuf) {
+        let dir = tempfile::TempDir::new().unwrap();
+        let path = dir.path().join("graph.json");
+        (dir, path)
     }
 
     async fn run(spawner: MockSpawner, graph_file: &Path) -> GraphPlannerOutcome {
@@ -216,8 +218,7 @@ mod tests {
 
     #[tokio::test]
     async fn valid_artifact_yields_canonical_nodes() {
-        let target = tmp_graph_file("valid");
-        let _ = std::fs::remove_file(&target);
+        let (_tmp, target) = tmp_graph_file("valid");
         let body = serde_json::json!({
             "nodes": [
                 {"id": "core", "title": "Core", "spec": "core spec", "deps": []},
@@ -242,8 +243,7 @@ mod tests {
 
     #[tokio::test]
     async fn prompt_embeds_objective_feedback_and_tool_names() {
-        let target = tmp_graph_file("prompt");
-        let _ = std::fs::remove_file(&target);
+        let (_tmp, target) = tmp_graph_file("prompt");
         let spawner = MockSpawner {
             response: MockReply::Done,
             body: None,
@@ -274,7 +274,7 @@ mod tests {
 
     #[tokio::test]
     async fn invalid_artifact_is_retryable_with_reason() {
-        let target = tmp_graph_file("invalid");
+        let (_tmp, target) = tmp_graph_file("invalid");
         let spawner = MockSpawner {
             response: MockReply::Done,
             body: Some(br#"{"nodes":[{"id":"a","title":"A","spec":"s","deps":["a"]}]}"#.to_vec()),
@@ -291,8 +291,7 @@ mod tests {
 
     #[tokio::test]
     async fn missing_artifact_fails_closed() {
-        let target = tmp_graph_file("missing");
-        let _ = std::fs::remove_file(&target);
+        let (_tmp, target) = tmp_graph_file("missing");
         let spawner = MockSpawner {
             response: MockReply::Done,
             body: None,
@@ -309,7 +308,7 @@ mod tests {
 
     #[tokio::test]
     async fn runtime_error_fails_closed() {
-        let target = tmp_graph_file("runtime");
+        let (_tmp, target) = tmp_graph_file("runtime");
         let spawner = MockSpawner {
             response: MockReply::Runtime { cancelled: false },
             body: None,
@@ -326,7 +325,7 @@ mod tests {
 
     #[tokio::test]
     async fn oversize_artifact_is_invalid_with_cap_in_reason() {
-        let target = tmp_graph_file("oversize");
+        let (_tmp, target) = tmp_graph_file("oversize");
         let mut body = vec![b'x'; (MAX_GRAPH_JSON_BYTES as usize) + 1];
         body[0] = b'{'; // content is irrelevant; the size gate fires first
         let spawner = MockSpawner {
@@ -345,7 +344,7 @@ mod tests {
 
     #[tokio::test]
     async fn transport_error_fails_closed() {
-        let target = tmp_graph_file("transport");
+        let (_tmp, target) = tmp_graph_file("transport");
         let spawner = MockSpawner {
             response: MockReply::Transport,
             body: None,
@@ -362,7 +361,7 @@ mod tests {
 
     #[tokio::test]
     async fn cancelled_runtime_error_reports_aborted() {
-        let target = tmp_graph_file("aborted");
+        let (_tmp, target) = tmp_graph_file("aborted");
         let spawner = MockSpawner {
             response: MockReply::Runtime { cancelled: true },
             body: None,