ci: Cut releases with release-please and lint PR titles

[mangas]

Adds release-please to cut releases from Conventional Commits, and a check that PR titles follow that format.

• .github/workflows/release.yml: on every push to master, release-please opens or updates a release PR. Merging that PR bumps the version in Cargo.toml and Cargo.lock, updates CHANGELOG.md, tags vX.Y.Z and creates the GitHub release.
  • It runs as a GitHub App, using secrets RELEASE_APP_CLIENT_ID and RELEASE_APP_PRIVATE_KEY, because PRs opened with GITHUB_TOKEN don't trigger CI.
• release-please-config.json and .release-please-manifest.json start from the v0.45.0 release commit (8e3debd67, v0.45.0 release #6703) at version 0.45.0.
  • bump-minor-pre-major is set, so below 1.0 feat and breaking changes bump the minor version, and fix and perf bump the patch.
• .github/workflows/semantic-pr.yml checks PR titles against the Conventional Commits types.
• CONTRIBUTING.md and CLAUDE.md move the commit format from crate: desc to type(crate): desc.

Decisions for maintainers before this leaves draft:

1. Merge method. release-please reads every commit on master. With rebase-merge, every commit in a PR must be conventional. With squash-merge, only the PR title must be, and that's what the title check enforces.
2. Changelog file. The changelog goes to a new CHANGELOG.md; NEWS.md stays hand-written. The alternative is to point changelog-path at NEWS.md.
3. Release binaries. Attaching binaries to the release is not part of this PR.
4. GitHub App. The App and its two secrets need creating.