fix(MM-144): address second round of PR review feedback
- Complete compensation path: session-token failure now also deletes
device-token to avoid orphaned credentials
- Extract map_409_subcode() helper; test calls the real function instead
of duplicating its logic
- HandleScreen: tighten handle regex to RFC 1035 DNS label (no dots or
underscores — these create multi-label handles or violate DNS spec)
- CLAUDE.md: document delete_item in both keychain listings
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