Fix bd-306: Use PID suffix for temp files to avoid concurrent collisions

- Change temp filename from issues.jsonl.tmp to issues.jsonl.tmp.<pid>
- Prevents race conditions when multiple bd commands run concurrently
- Added issues bd-300 through bd-306 (git-based restoration epic)

cmd/bd/main.go

@@ -634,7 +634,8 @@ func flushToJSONL() {
 	})
 
 	// Write to temp file first, then rename (atomic)
-	tempPath := jsonlPath + ".tmp"
+	// Use PID in filename to avoid collisions between concurrent bd commands (bd-306)
+	tempPath := fmt.Sprintf("%s.tmp.%d", jsonlPath, os.Getpid())
 	f, err := os.Create(tempPath)
 	if err != nil {
 		recordFailure(fmt.Errorf("failed to create temp file: %w", err))