fix(staleness): use RFC3339Nano precision for last_import_time (#399)

The staleness check compares last_import_time against JSONL file mtime.
File mtime has nanosecond precision, but last_import_time was stored with
only second precision (RFC3339). This caused a race condition where the
stored time could be slightly earlier than the file mtime, triggering
false "Database out of sync" errors - particularly in git worktrees.

Changed all 6 locations that set last_import_time to use RFC3339Nano.
The CheckStaleness parser already handles both formats, so this is
backward compatible.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

cmd/bd/daemon_sync.go

@@ -295,7 +295,8 @@ func updateExportMetadata(ctx context.Context, store storage.Storage, jsonlPath
 		log.log("Next export may require running 'bd import' first")
 	}
 
-	exportTime := time.Now().Format(time.RFC3339)
+	// Use RFC3339Nano for nanosecond precision to avoid race with file mtime (fixes #399)
+	exportTime := time.Now().Format(time.RFC3339Nano)
 	if err := store.SetMetadata(ctx, timeKey, exportTime); err != nil {
 		log.log("Warning: failed to update %s: %v", timeKey, err)
 	}