release: v0.12.0 — a trust release (the full audit, fixed)

[AshwinUgale]

Bumps to 0.12.0 and turns the [Unreleased] notes into ## [0.12.0] — 2026-09-28. The notes are grouped by area (trust core first) under an Upgrade notes: what may change for you summary. No line of the notes was dropped (checked line by line), and the history below is unchanged.

0.12.0 carries #56 and the six audit PRs (#57–#62).

Pre-release checks on this branch:

• pytest 627 passed; ruff / ruff format --check / mypy clean
• python -m build produces muteval-0.12.0 sdist (190 files, no .hypothesis) and wheel (40 files); twine check passes on both
• the built wheel installs into a clean venv, reports 0.12.0, and the keyless muteval init --template rag + muteval run runs end to end

After merge: tag v0.12.0 on the merge commit and push it. release.yml then publishes to PyPI via Trusted Publishing. Then check a clean-room pip install muteval==0.12.0.

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