Plan B: MCP state server + independent worker processes

[adbarc92]

Plan B — MCP state server + independent worker processes

Second of the engine build (Plan A landed in #5). This makes the "MCP-based state server" and "independent processes resolving execution order and shared state without collision" resume claims literally demonstrable, and brings the whole engine to CI-green.

What this adds (on top of Plan A's coordination core)

• state_server.py — a genuine FastMCP server (streamable-HTTP, mcp==1.28.1) wrapping the single-writer Store, running the reaper; launchable as python -m backend.engine.state_server (+ appforge_mcp_server.py alias).
• mcp_tools.py / client.py — the full MCP tool surface (create/claim/complete/heartbeat/fail/approve/get_state/put_state/get_run), JSON in/out; EngineClient MCP-client wrapper.
• agent_adapter.py — task→agent bridge via the existing registry (mock/real), including the Clarify Q&A loop that produces a PRD from mock agents autonomously.
• worker.py — the independent worker loop (claim → execute + background heartbeat → complete/fail), runnable as python -m backend.engine.worker.
• run.py — controller/CLI that boots the server, spawns N worker subprocesses, drives approval gates, and tears everything down leak-free.
• Read-only Store.snapshot.

Evidence

• Marquee multi-process demo (tests/engine/test_run_e2e.py): a full pipeline runs across 3 real OS worker subprocesses (distinct PIDs), traversing all six phases (Clarify→Design→Code→Test→Deploy→Iterate) to done.
• Two-process state sharing (test_server_state.py): two independent MCP clients share state through the server, incl. a CAS conflict.
• Every task went through TDD + an independent spec/quality review; the whole branch had a final opus whole-branch review (verdict: ready to merge, no Critical/Important).
• CI-green: ruff check 0 errors, black --check clean, pytest tests/ = 168 passed, 86.99% coverage (gate 70%).

Note on the diff

This PR also reformats the Plan A files already on main (store.py, scheduler.py, etc.) with black + ruff. Plan A merged (#5) before that cleanup existed, so main is currently failing black --check; merging this PR fixes it. The reformatting is style-only (verified behavior-preserving in review).

Deferred to Plan C (scoped out, not broken)

Live claim-time budget downgrade + a real-run assertion; the N≫workers concurrency stress test (the true collision-freedom proof); preserved documented-run artifacts under docs/runs/; reaper MAX_ATTEMPTS cap; LangGraph orchestrator retirement + its ~25 coupled tests; the Socket.IO events bridge; installing the repo so the bare appforge console script is on PATH (today python -m backend.engine.run run "<idea>" works).