refactor: rename last_import_hash to jsonl_content_hash (bd-39o)

The metadata key 'last_import_hash' was misleading because it's updated on
both import AND export. Renamed to 'jsonl_content_hash' which more accurately
describes its purpose - tracking the content hash of the JSONL file.

Added migration support: read operations try new key first, then fall back
to old key for backwards compatibility with existing databases.

Files modified:
- cmd/bd/integrity.go: Update key name with migration support
- cmd/bd/import.go: Update key name
- cmd/bd/sync.go: Update key name
- cmd/bd/autoflush.go: Update key name with migration support
- cmd/bd/daemon_sync.go: Update key name
- cmd/bd/daemon_event_loop.go: Update key name with migration support
- internal/autoimport/autoimport.go: Update key name with migration support
- Updated all related tests

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

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

cmd/bd/daemon_event_loop.go

@@ -174,10 +174,13 @@ func checkDaemonHealth(ctx context.Context, store storage.Storage, log daemonLog
 	// Health check 1: Verify metadata is accessible
 	// This helps detect if external operations (like bd import --force) have modified metadata
 	// Without this, daemon may continue operating with stale metadata cache
-	if _, err := store.GetMetadata(ctx, "last_import_hash"); err != nil {
-		log.log("Health check: metadata read failed: %v", err)
-		// Non-fatal: daemon continues but logs the issue
-		// This helps diagnose stuck states in sandboxed environments
+	// Try new key first, fall back to old for migration (bd-39o)
+	if _, err := store.GetMetadata(ctx, "jsonl_content_hash"); err != nil {
+		if _, err := store.GetMetadata(ctx, "last_import_hash"); err != nil {
+			log.log("Health check: metadata read failed: %v", err)
+			// Non-fatal: daemon continues but logs the issue
+			// This helps diagnose stuck states in sandboxed environments
+		}
 	}
 
 	// Health check 2: Database integrity check