ci: use a maintained commitlint action fork

[robinbowes]

What

Points the commitlint job at yo61/commitlint-github-action — a maintained fork running commitlint v21 on Node 24.

Why

Dependabot PRs fail the commitlint job on body-max-line-length, while the pre-commit lint job passes the same commit. Same commit, same commitlint.config.mjs, different verdict.

@commitlint/ensure v20 added an exemption for long lines containing a URL:

const URL_REGEX = /\bhttps?:\/\/\S+/;
value.split(/\r?\n/).every((line) => URL_REGEX.test(line) || ensure(line, max));

wagoid/commitlint-github-action@v6.2.1 is the latest release and pins @commitlint/ensure: ^19.0.3, so CI enforced a stricter rule than the config intends. Dependabot bodies are markdown link lists — one real example is 314 characters on a single line.

Upstream is unmaintained: no commit to master since 2025-01-14, and the v20 bump (PR #837) has been open and mergeable since July with no response.

The fork

yo61/commitlint-github-action@v6.3.0 — all @commitlint/* at ^21, Node 24.19.0 LTS (v21 needs node >= 22.12), commitlint-plugin-function-rules at ^5 (v4's peer range caps at <20 and breaks npm ci), published to ghcr.io from a pushed tag. Actions SHA-pinned, least-privilege permissions, actionlint and zizmor clean.

Verified against the action's own load+lint path with a real commitlint.config.mjs:

Case	Result
Dependabot commit, 314-char body line	valid
Sentence-case subject with subject-case: [0]	valid — the .mjs override is honored
Malformed commit	rejected

Commit subjects stay validated for Dependabot as well as humans, rather than exempting bot commits.