docs: add idle detection design for WindowWatcher

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

docs/plans/2026-02-27-idle-detection-design.md

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+# Idle Detection for WindowWatcher
+
+## Summary
+
+Add idle detection to WindowWatcher so it automatically pauses the active TaskTracker task when the user is away, and resumes it when they return.
+
+## Behavior
+
+- **Detection method:** `GetLastInputInfo` Win32 API — returns time of last keyboard/mouse input
+- **Idle threshold:** Configurable via `IdleTimeoutMs` (default: 300,000ms = 5 minutes)
+- **Checked on existing poll cycle** — no new timers or threads
+
+### State Machine
+
+```
+ACTIVE  ──(idle > threshold)──►  IDLE
+  ▲                                │
+  └──(input detected)──────────────┘
+```
+
+### On Idle Transition
+
+1. `GET /api/tasks/active` to find the current task
+2. `PUT /api/tasks/{id}/pause` with note "Auto-paused: idle timeout"
+3. Store paused task ID locally
+
+### On Resume Transition
+
+1. Check if stored task ID is still in Paused status
+2. If yes: `PUT /api/tasks/{id}/resume` with note "Auto-resumed: user returned"
+3. Clear stored task ID
+
+## Files Changed
+
+| File | Change |
+|------|--------|
+| `NativeMethods.cs` | Add `GetLastInputInfo` + `LASTINPUTINFO` struct |
+| `Worker.cs` | Add idle state tracking, check idle time each poll, call pause/resume APIs |
+| `WindowWatcherOptions.cs` | Add `IdleTimeoutMs` (default: 300000) |
+| `appsettings.json` | Add `IdleTimeoutMs` setting |
+
+## Edge Cases
+
+- **No active task when idle fires:** Log, skip pause, don't store task ID
+- **Task manually changed while idle:** On resume, verify stored task is still paused before resuming
+- **API unreachable:** Log warning, retry on next poll cycle. Maintain idle state locally
+
+## Out of Scope
+
+- Session lock/unlock detection
+- New context event types
+- Tray menu UI changes