sign release images with cosign, embed SBOMs, attach SLSA provenance (#10983)

* feat: sign release images with cosign and attach SBOM + SLSA provenance

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W8mi68bMNUFwCge7xAqyky

* fix: pin cosign-installer to exact version (no floating v4 tag exists)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W8mi68bMNUFwCge7xAqyky

* fix: embed SBOMs at build time via depot instead of rekor-bound cosign attest

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W8mi68bMNUFwCge7xAqyky

* docs: latest/main tags are only signed until the next main push

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W8mi68bMNUFwCge7xAqyky

* fix: gate signing on push events in cli/extra workflows, verify version tag

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W8mi68bMNUFwCge7xAqyky

* fix: refuse tag-targeted dispatches in publish workflows, use GITHUB_REF env

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W8mi68bMNUFwCge7xAqyky

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>