ci: attest builds and attach provenance

[av1155]

Summary

Produces a signed provenance attestation for each released image and puts it somewhere a user can actually reach it.

Closes #737

Changes

• docker.yml: add actions/attest-build-provenance against the pushed image digest, with push-to-registry: true.
• docker.yml: attach the Sigstore bundle to the GitHub release as houndarr-<tag>.sigstore.json.
• docker.yml: the build job gains id-token, attestations, and contents: write for the upload.

BuildKit already emitted provenance: mode=max and an SBOM, but those are registry-side only, so anyone verifying a release from GitHub had nothing to check against. Releases previously carried no assets at all.

release.yml is triggered by the same tag push but runs independently, so the upload waits for the release rather than assuming it beats a multi-arch build.

Testing

actionlint clean. New steps are gated on startsWith(github.ref, refs/tags/v), so PR builds are untouched; this PR exercises that path by not running them. Shell inputs are passed through env: rather than inline expressions.

Worth noting: the build job now holds contents: write. It is job-level rather than top-level, the steps using it are tag-gated, and a fork PR token is read-only regardless.

Type of Change

• CI/CD (ci:)

Checklist

• Issue exists and is linked above with Closes #N
• Linked issue has exactly one type:* and one priority:* label
• Linked issue has at most one phase:* label (or none when not roadmap work)
• Branch name matches issue scope
• Tests added/updated for all changes — N/A, no application code
• No secrets or sensitive data committed
• Scope check: release integrity only, no behaviour change