fix: improve session_end coverage for process.exit() and add dedup guard

Address review comment about beforeExit limitations:
- Add process.once("exit") handler to cover process.exit() calls
- Add dedup flag to prevent double-firing when finally block also runs
- Document why SIGINT/SIGTERM are NOT handled (abort controller already
  triggers loop exit → finally block → normal session_end)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: anandgupta42

packages/altimate-code/src/session/prompt.ts

@@ -303,7 +303,10 @@ export namespace SessionPrompt {
     const session = await Session.get(sessionID)
     await Telemetry.init()
     Telemetry.setContext({ sessionId: sessionID, projectId: Instance.project?.id ?? "" })
-    const beforeExitHandler = () => {
+    let emergencySessionEndFired = false
+    const emergencySessionEnd = () => {
+      if (emergencySessionEndFired) return
+      emergencySessionEndFired = true
       Telemetry.track({
         type: "session_end",
         timestamp: Date.now(),
@@ -314,7 +317,13 @@ export namespace SessionPrompt {
         duration_ms: Date.now() - sessionStartTime,
       })
     }
-    process.once("beforeExit", beforeExitHandler)
+    // beforeExit covers event-loop drain without entering the session loop.
+    // exit covers process.exit() calls (sync only — track() buffers, flush is best-effort).
+    // SIGINT/SIGTERM are NOT handled here: the abort controller already triggers
+    // loop exit → finally block → normal session_end. Adding signal handlers
+    // would interfere with the default termination behavior.
+    process.once("beforeExit", emergencySessionEnd)
+    process.once("exit", emergencySessionEnd)
     try {
     while (true) {
       SessionStatus.set(sessionID, { type: "busy" })
@@ -808,7 +817,8 @@ export namespace SessionPrompt {
     }
     SessionCompaction.prune({ sessionID })
     } finally {
-      process.removeListener("beforeExit", beforeExitHandler)
+      process.removeListener("beforeExit", emergencySessionEnd)
+      process.removeListener("exit", emergencySessionEnd)
       const outcome: "completed" | "abandoned" | "error" = abort.aborted
         ? "abandoned"
         : sessionHadError