chore(release): prepare Hangar v0.1.0-rc.28

[szymczag]

Summary

• prepare Hangar v0.1.0-rc.28 release notes and public version references
• document the signed, single-use Todoist preview grant and transactional duplicate admission
• add the rc.27 to rc.28 upgrade, migration, coordinated rollout, verification, and rollback procedure

Release boundary

This remains an AMD64 evaluation prerelease. It does not claim production support or close the production qualification gates.

The release includes migration ext.0011_import_job_preview_nonce. Existing rows require no backfill, but the migration must complete before traffic and the rc.28 web/API images must be deployed together.

Validation

• Oxfmt passed for all changed documentation
• 40 release metadata/workflow unit tests passed
• release metadata resolves hangar-v0.1.0-rc.28 as a prerelease
• curated release notes validated against the current GitHub release history and rc.27 baseline
• git diff --check passed