fix(release): retry signing for Hangar v0.1.0-rc.29

[szymczag]

Summary

• retry transient Cosign signing failures up to three times with bounded backoff for component images, AIO, and the Helm chart
• retain mandatory signature verification after signing and all immutable-release guards
• record rc.28 as an incomplete publication and prepare recovery release v0.1.0-rc.29

Root cause

The rc.28 API image build and provenance attestation completed, but Sigstore's timestamp service reset the connection during cosign sign. Matrix fail-fast canceled the remaining builds. Because immutable tags were partially published, rc.28 cannot be safely resumed or reused.

Impact

Transient Sigstore transport failures can recover without weakening identity verification. Persistent failures still stop publication after three attempts, and a partially published version remains immutable and unusable.

rc.29 remains an AMD64 evaluation prerelease. It includes the Todoist admission hardening and migration originally prepared for rc.28; operators upgrade directly from the last complete release, rc.27.

Validation

• 41 release workflow/metadata unit tests passed
• Oxfmt passed for workflow, tests, and release documentation
• curated rc.29 notes validated against the current GitHub release history
• release metadata resolves hangar-v0.1.0-rc.29 as a prerelease
• git diff --check passed