[rc1-prep] Add step-security/harden-runner to ko-publish for egress audit

[trilamsr]

Per docs/v1-rc1-operational-gaps.md §1 SLSA L3, remediation step 2.

Work: add step-security/harden-runner@<sha> to the ko-publish job in .github/workflows/release.yml with an egress allowlist pinned to proxy.golang.org, sum.golang.org, gcr.io, ghcr.io, fulcio.sigstore.dev, rekor.sigstore.dev.

Note: on ubuntu-latest, harden-runner audits egress rather than blocking it. This delivers an attestable network-trace artifact alongside SLSA provenance.

Acceptance: every release-tag run uploads a step-security/harden-runner audit artifact showing zero egress to undeclared endpoints.

Effort: S.

[trilamsr]

Addressed in PR #358 (ci(release): SLSA L3 provenance + harden-runner egress audit); closing.