feat: timer engine + full-screen timer UI

Implement the core timer feature following the src/features/ architecture:

- useTimerEngine hook: drift-free Date.now() delta countdown (100ms tick),
  explicit state machine (IDLE → GET_READY → WORK → REST → COMPLETE),
  event emitter for external consumers (PHASE_CHANGED, ROUND_COMPLETED,
  COUNTDOWN_TICK, SESSION_COMPLETE), auto-pause on AppState interruption
  (phone calls, background), expo-keep-awake during session
- TimerDisplay component: full-screen animated UI with 600ms color
  transitions between phases, pulse animation on countdown, flash red
  on last 3 seconds, round progress dots, IDLE/active/COMPLETE views
- TimerControls component: stop/pause-resume/skip buttons with Ionicons
- Timer route (app/timer.tsx): fullScreenModal wiring hook → display
- Home screen: dark theme with START button navigating to /timer
- Project docs: CLAUDE.md (constitution), PRD v1.1, skill files
- Shared constants: PHASE_COLORS, TIMER_DEFAULTS, formatTime utility
- Types: TimerPhase, TimerState, TimerConfig, TimerActions, TimerEvent

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

package-lock.json

@@ -17,6 +17,7 @@
         "expo-font": "~14.0.11",
         "expo-haptics": "~15.0.8",
         "expo-image": "~3.0.11",
+        "expo-keep-awake": "~15.0.8",
         "expo-linking": "~8.0.11",
         "expo-router": "~6.0.23",
         "expo-splash-screen": "~31.0.13",