fix(engine): stop tears down in-progress merger and triager sessions

TriageProcessor.stop() previously only halted the polling loop, so
in-flight specify sessions and their reviewer subagents kept streaming
past shutdown. Extracted the existing global-pause teardown into
abortAndDisposeActiveSessions() and call it from stop() too.

aiMergeTask creates three sessions during a merge — autostash resolver,
in-merge verification fix agent, and pull-rebase conflict resolver — but
only the autostash one was registered via onSession. The other two are
now registered (with onSession threaded through pushToRemoteAfterMerge
into the rebase resolver chain), so ProjectEngine.stop() actually
disposes whichever merger session is running when shutdown lands.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/src/merger.ts

@@ -785,6 +785,10 @@ Do not refactor, rename broadly, or make opportunistic improvements.
         taskTitle: taskForSkillContext?.title,
       }),
     });
+    // Register so engine.stop() can dispose this session — without this the
+    // fix agent keeps streaming past shutdown because it's not the autostash
+    // session that the engine tracks.
+    options.onSession?.(session);
 
     const runId = mergeRunContext?.runId;
     const agentId = mergeRunContext?.agentId ?? "merger";
@@ -2725,6 +2729,7 @@ async function resolveComplexRebaseConflictsWithAi(
     pluginRunner?: import("./plugin-runner.js").PluginRunner;
     signal?: AbortSignal;
     runtimeHint?: string;
+    onSession?: (session: { dispose: () => void }) => void;
   },
 ): Promise<void> {
   mergerLog.log(`${taskId}: resolving ${conflictedFiles.length} complex rebase conflict(s) with AI`);
@@ -2780,6 +2785,10 @@ You are assisting with a paused \`git pull --rebase\`.
       taskId,
     }),
   });
+  // Register so engine.stop() can dispose this session — without this, an
+  // in-progress rebase conflict resolution keeps streaming past shutdown
+  // (the engine only tracks the autostash session by default).
+  options?.onSession?.(session);
 
   const prompt = [
     `Resolve rebase conflicts for task ${taskId}.`,
@@ -2814,7 +2823,12 @@ async function resolveRebaseConflictSet(
   rootDir: string,
   taskId: string,
   settings: Settings,
-  options?: { onAgentText?: (delta: string) => void; signal?: AbortSignal; runtimeHint?: string },
+  options?: {
+    onAgentText?: (delta: string) => void;
+    signal?: AbortSignal;
+    runtimeHint?: string;
+    onSession?: (session: { dispose: () => void }) => void;
+  },
 ): Promise<void> {
   const conflictedFiles = await getConflictedFiles(rootDir);
   if (conflictedFiles.length === 0) return;
@@ -2857,7 +2871,12 @@ async function pullWithRebaseAndResolveConflicts(
   settings: Settings,
   remote: string,
   branch: string,
-  options?: { onAgentText?: (delta: string) => void; signal?: AbortSignal; runtimeHint?: string },
+  options?: {
+    onAgentText?: (delta: string) => void;
+    signal?: AbortSignal;
+    runtimeHint?: string;
+    onSession?: (session: { dispose: () => void }) => void;
+  },
 ): Promise<void> {
   const pullCommand = `git pull --rebase ${quoteArg(remote)} ${quoteArg(branch)}`;
   try {
@@ -2948,7 +2967,12 @@ export async function pushToRemoteAfterMerge(
   rootDir: string,
   taskId: string,
   settings: Settings,
-  options?: { onAgentText?: (delta: string) => void; signal?: AbortSignal; runtimeHint?: string },
+  options?: {
+    onAgentText?: (delta: string) => void;
+    signal?: AbortSignal;
+    runtimeHint?: string;
+    onSession?: (session: { dispose: () => void }) => void;
+  },
 ): Promise<{ pushed: boolean; error?: string }> {
   let target: { remote: string; branch: string };
 
@@ -4412,6 +4436,7 @@ export async function aiMergeTask(
         onAgentText: options.onAgentText,
         signal: options.signal,
         runtimeHint: pushRuntimeHint,
+        onSession: options.onSession,
       });
       if (pushResult.pushed) {
         mergerLog.log(`${taskId}: pushed merged result to remote`);