fix: reset hook state on abort so session recovers after user cancel

When a user cancels a generation (ESC x2), all three idle hooks could
enter a permanently broken state:

1. **todo-continuation-enforcer**: consecutiveFailures accumulated from
   abort-caused promptAsync failures, eventually hitting MAX_CONSECUTIVE_FAILURES
   and permanently stopping continuation injection.

2. **unstable-agent-babysitter**: no abort awareness at all — would keep
   firing reminders after user cancelled the session.

3. **runtime-fallback**: retry dedupe keys and pending fallback state
   persisted across cancellation, blocking legitimate error recovery.

Fix:
- Add shared `isAbortError()` utility for consistent abort detection
- Reset consecutiveFailures and clear stale state on AbortError in all hooks
- Track `lastCancelledAt` in todo-continuation-enforcer for abort window
- Add abort-awareness to unstable-agent-babysitter (skip if recently cancelled)
- Clear runtime-fallback retry state on abort errors

Tests: 61 pass, 0 fail across all 3 affected hook test suites.

Closes #2984

src/hooks/unstable-agent-babysitter/index.test.ts

@@ -181,4 +181,37 @@ describe("unstable-agent-babysitter hook", () => {
     expect(promptCalls.length).toBe(1)
     Date.now = originalNow
   })
+
+  test("skips follow-up reminder after the main session is cancelled", async () => {
+    setMainSession("main-1")
+    const promptCalls: Array<{ input: unknown }> = []
+    const ctx = createMockPluginInput({
+      messagesBySession: {
+        "main-1": [
+          { info: { agent: "sisyphus", model: { providerID: "openai", modelID: "gpt-4" } } },
+        ],
+        "bg-1": [
+          { info: { role: "assistant" }, parts: [{ type: "thinking", thinking: "deep thought" }] },
+        ],
+      },
+      promptCalls,
+    })
+    const backgroundManager = createBackgroundManager([createTask()])
+    const hook = createUnstableAgentBabysitterHook(ctx, {
+      backgroundManager,
+      config: { timeout_ms: 120000 },
+    })
+    const firstNow = Date.now()
+    const originalNow = Date.now
+    let currentNow = firstNow
+    Date.now = () => currentNow
+
+    await hook.event({ event: { type: "session.idle", properties: { sessionID: "main-1" } } })
+    await hook.event({ event: { type: "session.error", properties: { sessionID: "main-1", error: { name: "AbortError" } } } })
+    currentNow += 5 * 60 * 1000 + 1
+    await hook.event({ event: { type: "session.idle", properties: { sessionID: "main-1" } } })
+
+    expect(promptCalls.length).toBe(1)
+    Date.now = originalNow
+  })
 })