Create a real good-first-issue on-ramp (8-12 shallow issues)

[eaitbrahim]

The four currently open issues (#233, #259, #260, #198) are all deep: they require understanding the execution path, the backtest engine, or the broker port. There is no way for a newcomer to make a first contribution. A project with no shallow work gets no first-time contributors, regardless of how welcoming the README is.

Do

• Identify and file 8–12 genuinely shallow issues, labelled good first issue, each self-contained and reviewable in isolation. Candidates already visible:
  • a docs/ page for one existing rail, explaining it for a reader who is not the author
  • test coverage for a specific uncovered branch
  • a keel-broker-* adapter stub with the port implemented and every method raising NotImplementedError
  • CLI --help text improvements where wording is known to confuse
  • the .mypy_cache/ gitignore gap noted during review
• Each must state: the file to touch, what "done" looks like, and how to verify it
• Add help wanted to a second tier of medium issues

Anti-pattern to avoid

Do not label a deep issue good first issue to fill the list. A newcomer who picks one up and drowns does not come back.

[eaitbrahim]

Done — nine shallow issues filed, labelled good first issue, each self-contained with the file to touch, what "done" looks like, and how to verify:

• .gitignore: add .mypy_cache/ beside .ruff_cache/ and .pytest_cache/ #308 .gitignore: add .mypy_cache/ (gap verified — it is untracked at the root today)
• docs/rails/: explain rail 17 (§65.4 qabd / withdrawal capability) for a reader who is not the author #309 docs/rails/rail-17-qabd.md — §65.4 possession, explained for a non-author
• docs/rails/: explain rail 14 (subscription allowance) — the rail whose setting lives in the database, not config.yaml #310 docs/rails/rail-14-subscription-allowance.md — the database-not-config rail
• .env.example: say plainly that paper and simulate need a CDP key too — read-only is enough #311 .env.example: paper/simulate need a (read-only) CDP key — verified AuthenticationError without one
• keel monitor --help reads as a synonym of keel fetch --help — disambiguate the two #312 disambiguate keel monitor vs keel fetch --help (currently synonyms)
• packages/keel-broker-kraken: a stub adapter — port implemented, every method raising NotImplementedError #313 packages/keel-broker-kraken stub adapter, every method NotImplementedError
• docs/experiments/README.md: an index of the experiment record — 28 documents, no table of contents #314 docs/experiments/README.md index — 28 records, no table of contents
• docs/operations/purification.md: document the purification report (KB §65.9) #315 docs/operations/purification.md — document the KB §65.9 report
• test: pin that mypy's python_version matches requires-python, so the floor and the checker cannot drift #316 test: pin mypy python_version == requires-python floor (drift is silent since chore(python): relax the floor to >=3.11, measured, with a CI leg to keep it true #301)

All nine are in the keel project. None are deep — each is reviewable in isolation, and none
touch a rail or a default classification.

help wanted added to the medium tier: #259 (slippage as one global constant) and #233
(capability-based venue visibility). The four deep issues (#198, #260) stay unlabelled — the
anti-pattern this issue warns about is labelling something deep to fill a list.