Document hookable mail in all role templates and priming (gt-frhcq.4)

Add "Hookable Mail" section to all role templates explaining how mail beads
can be hooked for ad-hoc instruction handoff. Key documentation points:
- GUPP applies to hooked mail: read and execute instructions
- Two use cases: hook existing mail, create+hook via gt handoff
- Role-specific examples for each agent type

Templates updated:
- boot.md.tmpl
- crew.md.tmpl
- deacon.md.tmpl
- mayor.md.tmpl
- polecat.md.tmpl
- refinery.md.tmpl
- witness.md.tmpl

Also updated prime.go fallback output functions to include hookable mail
reference for when templates fail to render.

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

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

internal/templates/roles/mayor.md.tmpl

@@ -224,6 +224,21 @@ gt mol attach-from-mail <mail-id>
 
 Your hooked work persists across sessions. Handoff mail (🤝 HANDOFF subject) provides context notes.
 
+## Hookable Mail
+
+Mail beads can be hooked for ad-hoc instruction handoff:
+- `gt hook attach <mail-id>` - Hook existing mail as your assignment
+- `gt handoff -m "..."` - Create and hook new instructions for next session
+
+If you find mail on your hook (not a molecule), GUPP applies: read the mail
+content, interpret the prose instructions, and execute them. This enables ad-hoc
+tasks without creating formal beads.
+
+**Mayor use case**: The human can send you mail with high-level instructions
+(e.g., "prioritize security fixes across all rigs today"), then hook it. Your next
+session sees the mail on the hook and executes those instructions. Also useful for
+cross-session continuity when work doesn't fit neatly into a bead.
+
 ## Session End Checklist
 
 ```