Fix critical race conditions in auto-flush feature

Fixed three critical issues identified in code review:

1. Race condition with store access: Added storeMutex and storeActive
   flag to prevent background flush goroutine from accessing closed
   store. Background timer now safely checks if store is active before
   attempting flush operations.

2. Missing auto-flush in import: Added markDirtyAndScheduleFlush()
   call after import completes, ensuring imported issues sync to JSONL.

3. Timer cleanup: Explicitly set flushTimer to nil after Stop() to
   prevent resource leaks.

Testing confirmed all fixes working:
- Debounced flush triggers after 5 seconds of inactivity
- Immediate flush on process exit works correctly
- Import operations now trigger auto-flush
- No race conditions detected

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

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

cmd/bd/import.go

@@ -287,6 +287,9 @@ Behavior:
 			}
 		}
 
+		// Schedule auto-flush after import completes
+		markDirtyAndScheduleFlush()
+
 		// Print summary
 		fmt.Fprintf(os.Stderr, "Import complete: %d created, %d updated", created, updated)
 		if skipped > 0 {