fix: production readiness fixes for dependency trees and parallel agents

Critical fixes:
- Lock file TOCTOU race condition: Use atomic O_CREAT|O_EXCL for lock creation
- PID reuse vulnerability on Windows: Store PID:CREATE_TIME in lock file to
  detect when a different process has reused the same PID
- WAL mode on network drives: Detect network paths (UNC, mapped drives, NFS,
  CIFS) and fall back to DELETE journal mode to prevent corruption

High priority fixes:
- JSON migration now preserves dependencies field during legacy migration
- Process tree termination on Windows: Use psutil to kill child processes
  recursively to prevent orphaned browser instances
- Retry backoff jitter: Add random 30% jitter to prevent synchronized retries
  under high contention with 5 concurrent agents

Files changed:
- server/services/process_manager.py: Atomic lock creation, PID+create_time
- api/database.py: Network filesystem detection for WAL mode fallback
- api/migration.py: Add dependencies field to JSON migration
- parallel_orchestrator.py: _kill_process_tree helper function
- mcp_server/feature_mcp.py: Add jitter to exponential backoff

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

api/migration.py

@@ -83,6 +83,7 @@ def migrate_json_to_sqlite(
                 steps=feature_dict.get("steps", []),
                 passes=feature_dict.get("passes", False),
                 in_progress=feature_dict.get("in_progress", False),
+                dependencies=feature_dict.get("dependencies"),
             )
             session.add(feature)
             imported_count += 1