fix: align firehose event types with sync spec
- add missing CommitEvent fields: since, commit CID, blobs
- add cid to RepoOp for downstream CID verification
- make rev, time required on CommitEvent (spec: required)
- make time required on IdentityEvent and AccountEvent (spec: required)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>