fix(beads): preserve migrated comment records

[thinksyncs]

Summary

• Preserve comments when an approved schema migration changes only their IDs.
• Keep the snapshot guard fail-closed for content, timestamp, or multiplicity loss.

Changes

• Compare comment author, text, and creation time as a counted semantic fingerprint.
• Add positive ID-rekey and negative content, timestamp, and duplicate-collapse tests.
• Refresh the tracked snapshot from 117 to 125 remote-backed issues without removing dependencies or comments.

Testing

• PYTHONDONTWRITEBYTECODE=1 python3 scripts/test_beads_workflow.py
• Live guarded-export proof: 125 IDs, 70 dependencies, and 15 comments; candidate and guarded output SHA-256 matched.
• git diff --check

Notes

• The only changed common snapshot record is the known migrated comment ID; author, timestamp, text length, and text hash remain identical.
• No site content is changed.