Create AgentIdentity type to parse and construct session names, replacing
duplicated logic in sling.go and handoff.go.
- Add internal/session/identity.go with AgentIdentity type
- ParseSessionName handles: mayor, deacon, witness, refinery, crew, polecat
- SessionName() reconstructs valid tmux session name
- Address() returns mail-style address (e.g., "gastown/crew/max")
- GTRole() returns GT_ROLE env var format
- Update sling.go:sessionToAgentID to use ParseSessionName
- Update handoff.go:sessionToGTRole to use ParseSessionName
- Add comprehensive unit tests with round-trip verification
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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