Proposal: simplify the release/provenance machinery

[richardmhope]

Problem

The release and provenance tooling is large compared with what it ships:

• about 1,000 lines across seven scripts,
• 218 lines of provenance.py,
• a 228-line release workflow, and
• a near-duplicate of that pipeline in the CI build job.

All of this surrounds a ~1,000-line CLI with two runtime dependencies. Each extra step is something to keep working, and several don't add security, as described in the release-verification issue.

Proposed approach

This is for discussion, not a mandate.

Keep (these carry their weight):

• Trusted Publishing behind a protected environment.
• Build once, then publish the same bytes.
• SHA-pinned actions, Dependabot, and per-job permissions.
• build_plugin.py --check, check_release_consistency.py and prepare_release.py.
• One actions/attest-build-provenance call covering the wheel and sdist.

Candidates to remove or shrink:

• Replace verify_distribution_identity.py (403 lines) with build_plugin.py --check plus a short check that the wheel contains exactly the bundled skills.
• Remove the dual content manifest, with four hashes per skill and a domain-separated digest. The build attestation already binds the artifact digest to the commit.
• Remove _build_metadata.json stamping and stamp_build_metadata.py, for the same reason.
• Remove the SBOM step and verify_sbom.py, or keep one plain SBOM without attesting it. For two open-ranged dependencies it's a snapshot rather than a bill of materials (see the reproducibility issue).
• Merge the three attest calls into one.
• Remove the tamper step, and reduce verify-channels to one gh attestation verify plus a PyPI digest comparison.
• Replace the CI build job's copy of the release pipeline with a reusable workflow_call shared with release.yml. The two have already drifted: different uv cache settings and a different --tag check.

If the full machinery is intentional, for example as a reference implementation for #76, noting that in docs/releasing.md would settle the question.