Add model backend selector and Claude Agent SDK runner

[SakshiKekre]

Summary

• Adds a model backend abstraction for chat simulations with selectable UK compiled and UK Python backends
• Adds /chat/backends and a frontend Engine selector with backend package version display
• Keeps backend-specific prompts, tool descriptions, imports, and run_python execution environments behind the backend registry
• Adds an opt-in Claude Agent SDK runner behind POLICYENGINE_CHAT_AGENT_RUNNER=claude_sdk
• Exposes run_python to the SDK runner through an in-process MCP tool while preserving the existing frontend SSE event contract
• Adds Plan mode support so the next turn can ask clarifying questions before running tools
• Makes Modal install backend dependencies from backend/requirements.txt to avoid deployment dependency drift

Notes

The existing direct Anthropic Messages loop remains the default unless POLICYENGINE_CHAT_AGENT_RUNNER=claude_sdk is set. This lets us merge the backend selector work while keeping the SDK runner gated for comparison/testing.

Validation

• cd backend && ../.venv/bin/python -m py_compile routes/chatbot.py claude_agent_sdk_runner.py model_backends.py agent_tools.py ../modal_app.py
• cd backend && ../.venv/bin/python -m pytest tests/test_agent_tools.py tests/test_api.py::TestChatBackends::test_lists_backends tests/test_api.py::TestChatMessage::test_unknown_model_backend_returns_400
• cd frontend && npm run build
• Local SDK smoke tests with POLICYENGINE_CHAT_AGENT_RUNNER=claude_sdk:
  • simple text streaming returned chunk and done
  • Plan mode asked clarifying questions without running tools
  • tool-use prompt emitted tool_start, tool_use, tool_result, and final done

Manual test

Default runner:
cd backend && set -a; source ../.env; set +a; ../.venv/bin/python -m uvicorn main:app --host 127.0.0.1 --port 8002

SDK runner:
cd backend && set -a; source ../.env; set +a; POLICYENGINE_CHAT_AGENT_RUNNER=claude_sdk ../.venv/bin/python -m uvicorn main:app --host 127.0.0.1 --port 8001

Frontend examples:

• cd frontend && PORT=3001 BACKEND_URL=http://127.0.0.1:8002 npm run dev for default runner
• cd frontend && BACKEND_URL=http://127.0.0.1:8001 npm run dev for SDK runner

[vercel]

@SakshiKekre is attempting to deploy a commit to the PolicyEngine Team on Vercel.

A member of the Team first needs to authorize it.