fix: populate feature_flags.json and remove sys.path bootstrap inserts

- feature_flags.json: populated with 6 well-described flags reflecting
  current system state (DI migration, structlog, CORS enforcement,
  X-Wallet-Address trust, ZK proof, marketplace rate limiting)

- Remove sys.path.insert bootstrap from app main.py files:
  - apps/agent-management/src/app/main.py
  - apps/marketplace-service/src/marketplace_service/main.py
  - apps/wallet/src/app/main.py
  - apps/blockchain-node/src/aitbc_chain/combined_main.py
  PYTHONPATH is set correctly by the wrapper scripts in scripts/wrappers/
  so the inserts are redundant and misleading.

Wrapper scripts (scripts/wrappers/*.py) and lazy cross-app imports in
blockchain-event-bridge are intentionally left unchanged.

apps/agent-management/src/app/main.py

@@ -1,14 +1,6 @@
 #!/usr/bin/env python3
 """AITBC Agent Management Service"""
 
-import sys
-from pathlib import Path
-
-# Add project root to path
-project_root = Path(__file__).parent.parent.parent.parent.parent
-if str(project_root) not in sys.path:
-    sys.path.insert(0, str(project_root))
-
 import uvicorn
 from fastapi import FastAPI
 from aitbc import get_logger