Validate PR titles on creation, edits, and subsequent pushes in a small
read-only workflow. Pass the title as environment data, without checking out
or interpolating PR-controlled text as code. Cancel obsolete title runs.

Documentation impact:
Document the allowed types, optional scope, and breaking-change marker in
AGENTS, the development guide, and the PR template. Add the contributor
workflow to Unreleased. The title check will join required branch checks.

Verification:
- make check and make package passed.
- actionlint .github/workflows/pr-title.yml passed.
- Executed the workflow's Python validation against four valid and seven
  invalid titles; all returned the expected status.
- git diff --check passed.