docs: describe the release process (RELEASING.md)

[amiddavid]

Summary

Adds RELEASING.md: the release process, written for whoever takes the role of release integrator. It is the process used to cut v0.5.1 and v0.5.2. CONTRIBUTING.md links to it.

What changed

• New RELEASING.md, with these sections:
  1. Roles: release owner, integrator, PR author, reviewer.
  2. Plan: one release list, open PRs sorted into features, the owner picks.
  3. Review: the author's tests (a test that fails on main, mutations), one separate reviewer with 2 rounds by default, verdict in the first line, no live tests on single PRs, stop after the last round.
  4. Merge: squash with an explicit subject and body, merge at the approved head only, bypass only with the owner's authority, stacked PRs (rebase --onto, retarget before push, push with a lease), and defects found on the way go in their own PR.
  5. Freeze, then the integration check on the merged main: full race suites, and one isolated live session through a proxy built from main, with checks taken from the release list.
  6. Plugin version bump.
  7. Release notes: features and fixes only, [Notice] items, known limits, approved by the owner.
  8. Tag and publish.
  9. After the release.
  10. Notes for an integrator that is an AI agent session.
• CONTRIBUTING.md: one link to RELEASING.md under "Pull requests".

No code changes. The page names no test machines, credentials or internal hosts. It tells maintainers to keep those in their own agent instructions.

Open questions

• Should the page also go into the mkdocs site (for example under "Contributing & community")? It is at the repo root now, next to CONTRIBUTING.md, so it is not in the docs build.

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