Cleanup: format, lint, prune comments, fix tests, add CI

[derekwisong]

Summary

• Adopt black + ruff as dev deps; format pyagent/ and tests/.
• Strip narrative comments from pyagent/ (~11% line reduction with no behavior change).
• Prune stale planning docs and rewrite the survivors (docs/ loses ~56% of its lines); add docs/architecture.md with mermaid diagrams and a runnable examples/quickstart.py.
• Real bug fix: doc_tools._run_subllm was reading the wrong key on LLM responses (text → content), so extract_doc / summarize_doc were broken against every provider.
• Pin tree-sitter-language-pack <1.0 — the 1.x line moved most grammars (including Go) to download-on-demand and hung offline.
• Gate fastembed recall tests behind PYAGENT_HEAVY_TESTS=1 so CI doesn't try to download a ~130MB model.
• Update stale stubs in 5 smoke tests so the suite is green again (55/55 pass).
• Add a GitHub Actions workflow that runs black --check, ruff, and every smoke test on push to main and on PRs.

Diff: 123 files, +2,561 / −6,523 (~4,000 lines removed at no feature cost).

Test plan

• CI passes on this PR (lint + 55 smoke tests).
• Local: pip install -e '.[dev]' && python -m black --check pyagent tests && python -m ruff check pyagent tests.
• Local: for f in tests/smoke_*.py; do python -m tests.\$(basename \"\$f\" .py) || echo FAIL; done — expect every test to pass.
• Spot-check the rewritten docs — README.md, docs/architecture.md, docs/plugin-design.md, docs/library-usage.md.
• Verify examples/quickstart.py runs end-to-end with a real API key set.