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/federation.md

@@ -87,7 +87,8 @@ Distribute work across workspaces:
 
 ## Agent Provenance
 
-Every agent operation is attributed:
+Every agent operation is attributed. See [identity.md](identity.md) for the
+complete BD_ACTOR format convention.
 
 ### Git Commits