Add new `gt polecat identity` (alias: `id`) subcommand group with commands:
- add <rig> [name]: Create identity bead (auto-generates name if omitted)
- list <rig>: List polecat identity beads with session/worktree status
- show <rig> <name>: Show identity details and CV (work history)
- rename <rig> <old> <new>: Rename identity, preserving CV chain
- remove <rig> <name>: Remove identity with safety checks
Each command manipulates agent beads with role_type=polecat. Safety checks
prevent removal of identities with active sessions or work on hook.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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