docs(contributing): state the bar — documentation standard, gates, tests-first

[eaitbrahim]

Closes #282

The repository's documentation standard is its signature and its single biggest barrier to
contribution — nobody will guess it. An unstated bar filters for clairvoyance: PRs arrive at
ordinary quality, get heavy review, and the contributor quietly leaves.

What CONTRIBUTING.md now states

• The documentation standard, taught from a worked example: the _open_exposure_by_asset
  docstring in keel/execution/guards.py is quoted in full, then dissected on the three
  properties that make it acceptable — it says why, it names what was measured, and it
  says what it would take to change the decision.
• Dev setup and the gates: uv sync --all-extras --dev, uv run ruff check,
  uv run mypy, uv run pytest -q — pasteable, because a gate nobody can paste is a gate
  nobody runs.
• Tests first, with evidence: failing tests shown to fail for the right reason — an
  assertion, not an import error — carried in the PR.
• Conventional Commits, matching history; the prefix is load-bearing for releases.
• Three-tier scope: welcome / discuss-first (rails, default classifications, new deps,
  keel-broker-api surface) / out of scope (rulings as defaults, "make the bot profitable",
  weakening fails-closed paths).
• The honest-upfront framing the issue asks for: the bar is high, said so at the top, with
  why.
• Governance (rulings vs. machinery) and licence sections from Phase 6 kept intact.

Tests (tests/test_contributing.py, red first): gates stated verbatim; the worked
example present in CONTRIBUTING.md and still existing in guards.py (drift-proof, via
wrap/blockquote-normalized matching); the three properties named; tests-first + evidence;
commit convention; scope tiers.

Gates: pytest 2788 passed / 1 skipped, ruff check clean, mypy clean.