feat(daemon): add sync backoff and consolidate hints into tips

Daemon sync improvements:
- Adds exponential backoff on sync failures (30s → 1m → 2m → 5m → 10m → 30m cap)
- Tracks sync state in .beads/sync-state.json (NeedsManualSync, FailureCount, BackoffUntil)
- Resets backoff on daemon start and manual bd sync
- Adds sync-state.json to default .gitignore

Tips consolidation (following ox-cli pattern):
- Moves sync conflict hint from hints.go into tips.go
- Proactive health checks trump educational tips
- Uses InjectTip() with high priority (200) and 100% probability for urgent warnings
- Removes deprecated hints.go

cmd/bd/doctor/gitignore.go

@@ -19,6 +19,7 @@ daemon.lock
 daemon.log
 daemon.pid
 bd.sock
+sync-state.json
 
 # Local version tracking (prevents upgrade notification spam after git ops)
 .local_version