ci: skip the test matrix when no code paths change

[lan496]

Summary

• Add a changes job to ci.yaml using dorny/paths-filter, mirroring the pattern already in docs.yaml.
• Run the tests matrix only when code paths change: src/**, tests/**, pyproject.toml, uv.lock, .test_durations, or ci.yaml itself.
• Teach the ci gate job to accept a skipped tests job.

A docs-only pull request currently runs six test jobs (two Python versions x three splits). After this it runs none of them, while pre-commit still covers the change.

Why pre-commit stays unfiltered

pre-commit/action runs every hook over all files, so it is meaningful for any change including pure documentation. Leaving it ungated also guarantees the ci gate always has at least one real signal behind it rather than passing on an all-skipped workflow.

The gate change is the risky part

ci is the required status check in the repository's branch ruleset, so it must report success on a docs-only PR where tests is skipped. test "skipped" = success would fail and block every docs PR, hence:

case "${{ needs.tests.result }}" in
  success | skipped) ;;
  *) exit 1 ;;
esac

needs.changes.result is asserted separately. Without it, a failure in the changes job would skip tests, and the relaxed check would then read that skip as acceptable and pass the gate.

Test plan

• This PR touches only .github/workflows/ci.yaml, which is in the filter, so the full matrix should run and ci should pass
• Confirm on a docs-only PR (chore: rename release config and add CI/dependency sections #17 qualifies once rebased, or any follow-up) that tests is skipped and ci still reports success
• Confirm the ruleset's required ci check goes green in both cases

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