Implement timestamp tracking and staleness detection (bd-159, bd-160)

- bd-159: Track last_import_time in metadata after auto-import
- bd-160: Add staleness check to daemon before serving requests
  - Detects when JSONL mtime > last import time
  - Currently logs warning (needs bd-166 for actual import trigger)
- Add lastImportTime field to RPC Server struct with getter/setter
- Add checkAndAutoImportIfStale() method to detect stale JSONL

This is part of bd-158 epic to fix daemon showing stale data after git pull.

Amp-Thread-ID: https://ampcode.com/threads/T-b554e049-aff8-4a24-8bf3-3305483b7f5a
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/main.go

@@ -1099,6 +1099,12 @@ func autoImportIfNewer() {
 		fmt.Fprintf(os.Stderr, "Warning: failed to update last_import_hash after import: %v\n", err)
 		fmt.Fprintf(os.Stderr, "This may cause auto-import to retry the same import on next operation.\n")
 	}
+	
+	// Store import timestamp (bd-159: for staleness detection)
+	importTime := time.Now().Format(time.RFC3339)
+	if err := store.SetMetadata(ctx, "last_import_time", importTime); err != nil {
+		fmt.Fprintf(os.Stderr, "Warning: failed to update last_import_time after import: %v\n", err)
+	}
 }
 
 // checkVersionMismatch checks if the binary version matches the database version