mflux release prep

Releases are prepared in-repo; tagging/publishing is handled by GitHub Actions (release.yml, dispatched from main with the publish confirmation).

How notes work now (#685)

• Every PR carries a fenced ```release-note block in its body (CI enforces it; every PR gets a note, and contributors do not write none). That block is the only source of release notes.
• On dispatch, the ungated draft-notes job harvests the blocks for every PR whose squash commit is in previous-tag..HEAD and fills a DRAFT GitHub release, grouped by label.
• Whoever approves the pypi deployment reads and edits that draft (fixing any [needs edit] lines, adding contributor thanks if wanted); the gated job publishes it exactly as edited. A re-dispatch never overwrites an existing draft; delete the draft to re-harvest.
• There is no CHANGELOG.md: the notes live in GitHub Releases (https://github.com/mflux-community/mflux/releases).

Release-prep PR checklist

• Bump version in pyproject.toml
• Update lockfile: uv lock
• Release-note block of the prep PR itself: a note for contributors, for example Prepare the <version> release. Give the PR the chore label.
• Prefer one commit named release: prepare <version>
• Sanity checks (optional unless requested): just test-fast, just build
• Do not tag releases locally unless explicitly requested (normally handled by CI)