Add CONTRIBUTING with release runbook and CI overview

[frazane]

Adds a CONTRIBUTING guide documenting the CI and release setup, so the release process (which until recently had never actually run) is written down rather than living in people's heads.

What changed

• Development setup: how to install all backends, run the suite per-backend, and run the lint hooks.
• CI overview: what the test matrix and lint job do, the folded coverage leg, concurrency, and the repo automation (Dependabot, labeler, templates).
• Release runbook: the one-time Trusted Publisher / environment setup, the bare-tag convention (no v prefix, matching pyproject), the TestPyPI rehearsal, the supported release flow via OIDC, label-driven release notes, and a manual fallback.

Notes

• Branch protection is described as it actually is (one approving review; CI runs on PRs but no specific checks are pinned as required).
• Tag convention is bare X.Y.Z going forward; historical v-prefixed tags are left as-is.