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_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