feat(#39): stable surface — SQLite queue, extras gate, drop redis/cluster

[hadamrd]

Closes #39.

Summary

Quarantine experimental features behind pip install 'forge-loop[experimental]',
swap the Redis-backed queue + leader-election cluster code for a durable
SQLite-WAL queue, and document the stability matrix.

The product is one operator on one box; the default install now reflects
that. Everything else is opt-in.

Acceptance criteria → evidence

1. Stability matrix in README ✅
   README.md now opens with a STABLE / EXPERIMENTAL / REMOVED table
   that maps every module to its tier.
2. pyproject.toml extras ✅
   Default deps shrunk to: mcp, pyyaml, duckdb, anyio, anthropic,
   claude-agent-sdk. New [experimental] extra carries fastapi,
   uvicorn, prometheus_client, opentelemetry-*, jinja2. The redis +
   observability extras are gone. Each experimental module top-imports
   forge_loop._extras.require_experimental(...) — a clear ImportError
   names the extra to install if not present (override via
   FORGE_LOOP_EXPERIMENTAL=1 for local dev).
3. SQLite queue replaces Redis ✅
   • forge_loop.queue.sqlite.SQLiteQueue — WAL mode, FIFO via
     enqueued_at, ack/nack with head-bias on requeue, lock-guarded
     for concurrent producers. New production default.
   • InMemoryQueue stays as the test default.
   • forge_loop.queue.redis_backend and forge_loop.cluster.*
     deleted; build_queue('redis://...') raises a clear ValueError
     pointing to SQLite.
4. Critic prompt patch ✅
   critic.md.tmpl now lists product as a category and teaches the
   no-scaffold-theatre rule (sev2 finding for orphan backends /
   adapters). critic.VALID_CATEGORY accepts the new category.
5. README "running on a Claude subscription" ✅
   Spelled out: per-token budget tracking is unsupported in flat-fee
   mode; wall-clock + cooldowns are the safety nets.

Test matrix

• tests/test_queue_sqlite.py — happy-path FIFO, ack/nack semantics,
  timeout returns None, durability across reopen, attempts counter,
  concurrent multi-thread pop with no double-delivery, factory
  routing for sqlite/memory/redis URLs.
• tests/test_install_surface.py — every stable module imports clean;
  every experimental module raises ImportError when the gate is
  forced off; removed modules stay removed (regression guard); runner
  source contains no ClusterCoordinator / redis_backend / cluster
  import; critic accepts product; pyproject defaults contain none of
  fastapi/uvicorn/prometheus_client/redis/jinja2.
• tests/conftest.py — sets FORGE_LOOP_EXPERIMENTAL=1 so the
  existing experimental-module test files keep importing; the
  install-surface tests clear it via monkeypatch when simulating a
  default install (save/restore sys.modules so they don't pollute
  later tests' class identities).

Full suite: 413 passed, 1 skipped (skip is the live SDK integration
that needs an API key). ruff check src/ tests/ clean.

Test plan

• uv run pytest tests/test_queue_sqlite.py tests/test_install_surface.py
• uv run pytest tests/ (full suite stays green)
• uv run ruff check src/ tests/

🤖 Generated with Claude Code