fix(sync): make snapshot protection timestamp-aware (GH#865)

The --protect-left-snapshot mechanism was protecting ALL local issues
by ID alone, ignoring timestamps. This caused newer remote changes to
be incorrectly skipped during cross-worktree sync.

Changes:
- Add BuildIDToTimestampMap() to SnapshotManager for timestamp-aware
  snapshot reading
- Change ProtectLocalExportIDs from map[string]bool to map[string]time.Time
- Add shouldProtectFromUpdate() helper that compares timestamps
- Only protect if local snapshot is newer than incoming; allow update
  if incoming is newer

This fixes data loss scenarios where:
1. Main worktree closes issue at 11:31
2. Test worktree syncs and incorrectly skips the update
3. Test worktree then pushes stale open state, overwriting mains changes

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/import.go

@@ -262,19 +262,19 @@ NOTE: Import requires direct database access and does not work with daemon mode.
 			OrphanHandling:             orphanHandling,
 		}
 
-		// If --protect-left-snapshot is set, read the left snapshot and build ID set
-		// This protects locally exported issues from git-history-backfill
+		// If --protect-left-snapshot is set, read the left snapshot and build timestamp map
+		// GH#865: Use timestamp-aware protection - only protect if local is newer than incoming
 		if protectLeftSnapshot && input != "" {
 			beadsDir := filepath.Dir(input)
 			leftSnapshotPath := filepath.Join(beadsDir, "beads.left.jsonl")
 			if _, err := os.Stat(leftSnapshotPath); err == nil {
 				sm := NewSnapshotManager(input)
-				leftIDs, err := sm.BuildIDSet(leftSnapshotPath)
+				leftTimestamps, err := sm.BuildIDToTimestampMap(leftSnapshotPath)
 				if err != nil {
 					fmt.Fprintf(os.Stderr, "Warning: failed to read left snapshot: %v\n", err)
-				} else if len(leftIDs) > 0 {
-					opts.ProtectLocalExportIDs = leftIDs
-					fmt.Fprintf(os.Stderr, "Protecting %d issue(s) from left snapshot\n", len(leftIDs))
+				} else if len(leftTimestamps) > 0 {
+					opts.ProtectLocalExportIDs = leftTimestamps
+					fmt.Fprintf(os.Stderr, "Protecting %d issue(s) from left snapshot (timestamp-aware)\n", len(leftTimestamps))
 				}
 			}
 		}

cmd/bd/import_shared.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"time"
 
 	"github.com/steveyegge/beads/internal/importer"
 	"github.com/steveyegge/beads/internal/storage"
@@ -165,7 +166,7 @@ type ImportOptions struct {
 	SkipPrefixValidation       bool              // Skip prefix validation (for auto-import)
 	ClearDuplicateExternalRefs bool              // Clear duplicate external_ref values instead of erroring
 	OrphanHandling             string            // Orphan handling mode: strict/resurrect/skip/allow (empty = use config)
-	ProtectLocalExportIDs      map[string]bool   // IDs from left snapshot to protect from git-history-backfill (bd-sync-deletion fix)
+	ProtectLocalExportIDs      map[string]time.Time // IDs from left snapshot with timestamps for timestamp-aware protection (GH#865)
 }
 
 // ImportResult contains statistics about the import operation

cmd/bd/snapshot_manager.go

@@ -289,6 +289,48 @@ func (sm *SnapshotManager) BuildIDSet(path string) (map[string]bool, error) {
 	return sm.buildIDSet(path)
 }
 
+// BuildIDToTimestampMap reads a JSONL file and returns a map of issue ID to updated_at timestamp.
+// This is used for timestamp-aware snapshot protection (GH#865): only protect local issues
+// if they are newer than incoming remote versions.
+func (sm *SnapshotManager) BuildIDToTimestampMap(path string) (map[string]time.Time, error) {
+	result := make(map[string]time.Time)
+
+	// #nosec G304 -- snapshot file path derived from internal state
+	f, err := os.Open(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return result, nil // Empty map for missing files
+		}
+		return nil, err
+	}
+	defer f.Close()
+
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		line := scanner.Text()
+		if line == "" {
+			continue
+		}
+
+		// Parse ID and updated_at fields
+		var issue struct {
+			ID        string    `json:"id"`
+			UpdatedAt time.Time `json:"updated_at"`
+		}
+		if err := json.Unmarshal([]byte(line), &issue); err != nil {
+			return nil, fmt.Errorf("failed to parse issue from line: %w", err)
+		}
+
+		result[issue.ID] = issue.UpdatedAt
+	}
+
+	if err := scanner.Err(); err != nil {
+		return nil, err
+	}
+
+	return result, nil
+}
+
 // Private helper methods
 
 func (sm *SnapshotManager) createMetadata() snapshotMetadata {