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/runtime-fallback/event-handler.test.ts

@@ -104,4 +104,62 @@ describe("createEventHandler", () => {
     expect(abortCalls).toEqual([])
     expect(state.pendingFallbackModel).toBe(undefined)
   })
+
+  it("#given a cancelled session #when session.error receives an abort error #then fallback retry state is reset", async () => {
+    const sessionID = "session-cancelled"
+    const deps = createDeps()
+    const abortCalls: string[] = []
+    const clearCalls: string[] = []
+    const state = createFallbackState("google/gemini-2.5-pro")
+    state.currentModel = "openai/gpt-5.4"
+    state.fallbackIndex = 1
+    state.attemptCount = 2
+    state.pendingFallbackModel = "openai/gpt-5.4"
+    state.failedModels.set("google/gemini-2.5-pro", Date.now())
+    deps.sessionStates.set(sessionID, state)
+    deps.sessionRetryInFlight.add(sessionID)
+    deps.sessionAwaitingFallbackResult.add(sessionID)
+    deps.sessionStatusRetryKeys.set(sessionID, "retry:2")
+    const handler = createEventHandler(deps, createHelpers(deps, abortCalls, clearCalls))
+
+    await handler({ event: { type: "session.error", properties: { sessionID, error: { name: "AbortError" } } } })
+
+    const resetState = deps.sessionStates.get(sessionID)
+    expect(resetState?.originalModel).toBe("google/gemini-2.5-pro")
+    expect(resetState?.currentModel).toBe("google/gemini-2.5-pro")
+    expect(resetState?.fallbackIndex).toBe(-1)
+    expect(resetState?.attemptCount).toBe(0)
+    expect(resetState?.pendingFallbackModel).toBe(undefined)
+    expect(resetState?.failedModels.size).toBe(0)
+    expect(deps.sessionRetryInFlight.has(sessionID)).toBe(false)
+    expect(deps.sessionAwaitingFallbackResult.has(sessionID)).toBe(false)
+    expect(deps.sessionStatusRetryKeys.has(sessionID)).toBe(false)
+    expect(clearCalls).toEqual([sessionID])
+    expect(abortCalls).toEqual([])
+  })
+
+  it("#given a cancelled session #when session.idle fires #then fallback retry state stays cleared", async () => {
+    const sessionID = "session-cancelled-idle"
+    const deps = createDeps()
+    const abortCalls: string[] = []
+    const clearCalls: string[] = []
+    const state = createFallbackState("google/gemini-2.5-pro")
+    state.currentModel = "openai/gpt-5.4"
+    state.fallbackIndex = 1
+    state.attemptCount = 2
+    state.pendingFallbackModel = "openai/gpt-5.4"
+    deps.sessionStates.set(sessionID, state)
+    const handler = createEventHandler(deps, createHelpers(deps, abortCalls, clearCalls))
+
+    await handler({ event: { type: "session.error", properties: { sessionID, error: { name: "MessageAbortedError" } } } })
+    clearCalls.length = 0
+
+    await handler({ event: { type: "session.idle", properties: { sessionID } } })
+
+    const resetState = deps.sessionStates.get(sessionID)
+    expect(resetState?.currentModel).toBe("google/gemini-2.5-pro")
+    expect(resetState?.attemptCount).toBe(0)
+    expect(clearCalls).toEqual([sessionID])
+    expect(abortCalls).toEqual([])
+  })
 })