Document BD_ACTOR format convention (gt-6r18e.9)

Add docs/identity.md with canonical BD_ACTOR format:
- Town level: mayor, deacon
- Rig level: {rig}/witness, {rig}/refinery
- Workers: {rig}/crew/{name}, {rig}/polecats/{name}

Also documents the attribution model:
- GIT_AUTHOR_NAME = BD_ACTOR (agent identity)
- GIT_AUTHOR_EMAIL = overseer email (work owner)
- created_by = BD_ACTOR (beads field)

Updates reference.md with expanded environment variables section
and adds cross-reference from federation.md.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/reference.md

@@ -187,9 +187,15 @@ gt mol step done <step>      # Complete a molecule step
 
 | Variable | Purpose |
 |----------|---------|
+| `BD_ACTOR` | Agent identity for attribution (see [identity.md](identity.md)) |
 | `BEADS_DIR` | Point to shared beads database |
 | `BEADS_NO_DAEMON` | Required for worktree polecats |
+| `GIT_AUTHOR_NAME` | Set to BD_ACTOR for commit attribution |
+| `GIT_AUTHOR_EMAIL` | Workspace owner email |
 | `GT_TOWN_ROOT` | Override town root detection |
+| `GT_ROLE` | Agent role type (mayor, polecat, etc.) |
+| `GT_RIG` | Rig name for rig-level agents |
+| `GT_POLECAT` | Polecat name (for polecats only) |
 
 ## CLI Reference