ci(commitlint): 470 — gate Conventional Commits type at PR time

[randomdevpete]

Summary

Adds commitlint enforcement at PR time so untyped and miscased commits cannot reach master. The CI job runs only on pull_request events and checks the exact range the PR adds, using @commitlint/config-conventional's type-enum, type-case and type-empty rules — the same type set semantic-release's commit-analyzer recognises.

Style exceptions left standing

Four rules from @commitlint/config-conventional are disabled in commitlint.config.cjs because they would flag this project's normal prose patterns rather than genuine issues:

• header-max-length: The longest current commit header is 101 characters (e.g. refactor(demos): source is always shown - drop the accordion, add a filename heading linked to GitHub). Disabling lets legitimate long subjects through.
• body-max-line-length / footer-max-line-length: Disabled to allow long lines when quoting documentation or file paths.
• subject-case: Disabled to allow proper nouns naturally appearing in subjects (e.g. feat(foo): Add GitHub support — the capitalised "GitHub" would otherwise violate lowercase enforcement).

No local commit-msg hook

Deliberately omitted. The branch-base.sh init hook in <root>/TODOS/scripts/ installs a shared commit-msg hook that appends the Ticket: trailer, required for fork-point tracking in every worktree. Composing a second commitlint hook locally was judged riskier than it is worth, and every landing goes through a PR anyway, where the CI job gates the format. The hook remains in CI only.

Producer's validation

• Last 100 commits on origin/master: 89/100 pass conventionalcommits type rules. After the four rule disablements, only 5 genuinely untyped/miscased historical commits fail — exactly the problem this ticket names. These predate enforcement and will never be rewritten.
• All 29 commits on the 13 open PR branches pass (including stacked ones feat(docs): 676 — demo page for cancelling navigation on dirty edits #96–test(docs): 472 — give the docs package a test runner, and fix the fenced-heading bug it found #105). This confirms the rule set matches what this project already does.
• Board-mandated commit shape passes: fix(paper-kit): 760 — state the flat-piece origin and require flat pieces for surfaces ✓
• semantic-release's generated commit passes: chore(release): 2.6.0 [skip ci] ✓