Cleans up two workarounds in code_artifact.go that existed only because libfossil v0.4.x didn't expose them as first-class APIs:

- Delete the ridToUUID helper (raw SQL probe of libfossil's blob table). Replaced with r.UUIDFromRID(rid) at both callsites: Agent.Tip and Agent.toSyncResult.

- Collapse Agent.Diff from ~25 LOC of file enumeration to a single r.Diff(ridA, ridB, "") call. Uses libfossil v0.5.0's Option-A whole- checkin form. Also fixes wrong-ish rename-detection in the previous hand-rolled implementation.

Adds TestAgent_Diff_RenameAcrossRevs to exercise the rename case the old enumeration got wrong.

Closes #112 check-in: ab2db30284 user: [email protected] tags: trunk