fix: release workflow improvements

[walter-square]

Two related changes to release.yml, in two commits.

1. Build-provenance attestation + SHA256SUMS

• Adds actions/attest-build-provenance@v2 (digest-pinned) in the build job, emitting a SLSA v1.0 provenance statement signed by Sigstore via the runner's OIDC identity. Downstream installers can verify with gh attestation verify --owner block --repo lefthookmerge <file>.
• Generates SHA256SUMS over the .bz2 artifacts and attaches it to the release.
• Adds an explicit permissions block to the build job (contents: read, id-token: write, attestations: write) — setting any permission switches the job to a strict allowlist, so contents: read is restated to keep checkout working.

2. Tag-trigger glob + quoting

• The tag filter v[0-9]+.[0-9]+.[0-9]+ is a glob, not a regex — the literal + made it match only the tag v+.+.+. Releases fire today only because workflow_dispatch is also enabled. Replaced with v[0-9]*.[0-9]*.[0-9]*.
• Routes github.ref_name through a TAG env var and uses quoted "$TAG" in gh release create, so the value is always a single argument regardless of content.

Happy to split into two PRs if you'd rather review them separately.