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/main.go

@@ -864,6 +864,9 @@ var rootCmd = &cobra.Command{
 				debug.Logf("loaded %d molecules from %v", result.Loaded, result.Sources)
 			}
 		}
+
+		// Tips (including sync conflict proactive checks) are shown via maybeShowTip()
+		// after successful command execution, not in PreRun
 	},
 	PersistentPostRun: func(cmd *cobra.Command, args []string) {
 		// Handle --no-db mode: write memory storage back to JSONL