fix: cancel trailing async jobs on session switch to prevent wasted LLM turns (#1643)

When a unit spawns background jobs via async_bash, job completion callbacks
fire follow-up messages after agent_end has resolved. The auto-loop has
moved on but the previous session's LLM processes these follow-ups, adding
12-45s of wasted time and ~14 unnecessary turns per unit.

Two complementary fixes:
1. Cancel all running background jobs on session_before_switch so
   completion callbacks never fire for the old session.
2. Clear the follow-up queue after runUnit() completes as defense-in-depth,
   discarding any already-queued notifications before the next session starts.

Closes #1642

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/resources/extensions/async-jobs/index.ts

@@ -78,6 +78,17 @@ export default function AsyncJobs(pi: ExtensionAPI) {
 		});
 	});
 
+	pi.on("session_before_switch", async () => {
+		if (manager) {
+			// Cancel all running background jobs — their results are no longer
+			// relevant to the new session and would produce wasteful follow-up
+			// notifications that trigger empty LLM turns (#1642).
+			for (const job of manager.getRunningJobs()) {
+				manager.cancel(job.id);
+			}
+		}
+	});
+
 	pi.on("session_shutdown", async () => {
 		if (manager) {
 			manager.shutdown();

src/resources/extensions/gsd/auto-loop.ts

@@ -287,6 +287,20 @@ export async function runUnit(
     status: result.status,
   });
 
+  // Discard trailing follow-up messages (e.g. async_job_result notifications)
+  // from the completed unit. Without this, queued follow-ups trigger wasteful
+  // LLM turns before the next session can start (#1642).
+  // clearQueue() lives on AgentSession but isn't part of the typed
+  // ExtensionCommandContext interface — call it via runtime check.
+  try {
+    const cmdCtxAny = s.cmdCtx as Record<string, unknown> | null;
+    if (typeof cmdCtxAny?.clearQueue === "function") {
+      (cmdCtxAny.clearQueue as () => unknown)();
+    }
+  } catch {
+    // Non-fatal — clearQueue may not be available in all contexts
+  }
+
   return result;
 }