build: pin ruff exactly and exclude Markdown from the formatter

[Olen]

Summary

Pins ruff to ==0.16.3 and excludes *.md from the formatter.

Why pin

ruff 0.16 began formatting Python code blocks inside Markdown; 0.15 did not. With a >=0.15.0 floor and fresh resolution in CI, that turned #246 red without any change to the branch — the same commit passed in May and failed in August.

A floating formatter means a commit's CI verdict is not stable over time. On a repo with 41 forks that lands asymmetrically: an outside contributor's PR goes red for something unrelated to their change, and the fix gets bundled into whatever work happened to be in flight.

Pinning does not stall upgrades. An exact pin is precisely the case where Dependabot opens a PR — a >= floor gives it nothing to edit, which is why it has never proposed a ruff bump. So new versions still arrive and still flag latent issues; they just arrive as their own reviewable change.

Dev-only dependency, so nothing is imposed on downstream consumers.

Why exclude Markdown

Doc snippets are illustrative rather than compiled, and the formatter collapses hand-aligned trailing comments that make the examples scannable:

    SpondError,             # base — catch this for any SDK error    ← before
    SpondError,  # base — catch this for any SDK error               ← after

Scoped to [tool.ruff.format], so linting is unaffected.

Testing

• poetry run ruff format --check → 12 files (was 13; README.md now excluded)
• poetry run ruff check → all checks passed
• poetry run pytest → 30 passed

🤖 Generated with Claude Code