Run RepoPilot on the LangGraph runtime

[vcvyg]

What changed

• replace the custom graph executor with a LangGraph-backed runtime used by KnowledgeAgent.ask, SSE streaming, and debug endpoints
• use InMemorySaver, dynamic interrupt(), and Command(resume=...) for checkpoint/resume/cancel
• preserve Evidence Gate, bounded retrieval retries, ReAct tool execution, event traces, and API compatibility
• remove four duplicate adapter/bridge modules that were not connected to the main agent path
• update LangGraph to 1.2.11 and document the actual runtime boundary

Verification

• python -m pytest -q — 46 passed
• python -m ruff check app scripts tests — passed
• python -m mypy — passed
• offline evaluation — 7/7 passed; intent/source/tool/evidence-gate accuracy all 100%