fix: address systems review findings for agent-not-found fallback

1. [HIGH] Tool restrictions recomputed for fallback agent via
   buildFallbackBody() — no longer inherits original agent's restrictions.

2. [HIGH] Double-retry race prevented — handleSessionErrorEvent now
   returns early for agent-not-found errors, since the prompt catch
   block already handles them with agent fallback. This prevents
   tryFallbackRetry from racing with a model-level retry on the same
   error (the "not found" pattern in RETRYABLE_MESSAGE_PATTERNS).

3. [MEDIUM] task.agent updated to FALLBACK_AGENT after successful
   fallback — notifications, toast, and logging reflect actual agent.

4. [MEDIUM] FALLBACK_AGENT exported from spawner.ts and imported into
   manager.ts — single source of truth.

5. [LOW] resumeTask fallback now uses promptWithModelSuggestionRetry
   (consistent with startTask), getting timeout protection.

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

src/features/background-agent/spawner.test.ts

@@ -76,6 +76,14 @@ describe("background-agent spawner agent-not-found fallback", () => {
     expect(promptCalls[1].body.agent).toBe("general")
     // Original prompt content preserved in fallback
     expect(promptCalls[1].body.parts).toEqual(promptCalls[0].body.parts)
+    // Tool restrictions recomputed for fallback agent (general has no restrictions)
+    expect(promptCalls[1].body.tools).toEqual({
+      task: false,
+      call_omo_agent: true,
+      question: false,
+    })
+    // Task agent identity updated to reflect fallback
+    expect(task.agent).toBe("general")
     // Task should not have errored
     expect(onTaskError).not.toHaveBeenCalled()
   })