ci: Add commit-lint enforcement for PR titles

[scouten-adobe]

Summary

• Add .commitlintrc.yml and .github/workflows/pr_title.yml, ported from c2pa-rs's commit-lint setup.
• Unlike c2pa-rs, scope is disallowed entirely (optional: false, options: []), following adobe/xmp-toolkit-rs's no-scope pattern — c2patool is a single crate, so a scope would never disambiguate anything the way it does in c2pa-rs's multi-crate monorepo.
• The allowed type list matches c2pa-rs's (feat, fix, chore, docs, build, ci, perf, refactor, revert, style, test, update), since this repo's actual commit history (inherited from the c2pa-rs monorepo split) already uses most of these.
• Update docs/release-process.md with a "Commit lint used for PR title enforcement" section (mirroring c2pa-rs's, minus scope), replacing the stale note that flagged this as not-yet-ported follow-up work.
• Update CONTRIBUTING.md's "Pull request titles" section, which was still copied from c2pa-rs and described scope as optional-but-preferred — now points at this repo's own .commitlintrc.yml and states scope is disallowed.

Test plan

• Installed commitlint-rs locally and validated .commitlintrc.yml against sample titles: a valid no-scope title passes; a scoped title, a lowercase description, an unknown type, and a trailing period each correctly fail.
• Verified the workflow's chore: release/update: bump/update: update exception patterns match the title shapes release-plz and Dependabot actually produce.
• Ran actionlint against the new workflow (only a pre-existing-style shellcheck info note, matching what ci.yml already has).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.16%. Comparing base (1a771a8) to head (6af9ddd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #315   +/-   ##
=======================================
  Coverage   72.16%   72.16%           
=======================================
  Files           4        4           
  Lines        1060     1060           
=======================================
  Hits          765      765           
  Misses        295      295           

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