ci: enforce Conventional Commit PR titles

[gabrielfruet]

Pull requests are squash-merged, so the PR title is the commit subject that lands on master. This makes that title follow Conventional Commits and adds a CI check that rejects titles that don't match.

First of two PRs. This one only enforces the convention; the release automation that consumes it is the stacked follow-up (#2057).

Changes:

• lint_pr_title.yml validates the title against the allowed types on every PR, with a merge_group pass-through so the merge queue doesn't stall.
• .pre-commit-config.yaml gains a commit-msg hook running commitizen locally. Best-effort: squash discards local commit messages, so the CI check is the real gate.
• CONTRIBUTING.md, CLAUDE.md, and the PR templates document the types and the type→version-bump mapping.

Before making the check required:

• Set the squash-merge default to "Pull request title", not the first commit.
• Confirm the merge_group pass-through keeps the queue green.

No history is rewritten; only new PRs are affected.

[coderabbitai]

Important

Draft PR not reviewed

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• Trigger a manual review

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reviews:
  auto_review:
    drafts: true

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