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/witness.md.tmpl

@@ -176,6 +176,21 @@ bd update <wisp-id> --status=hooked --assignee={{ .RigName }}/witness
 
 **Work hooked → Execute. No exceptions.**
 
+## 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 patrol wisp), GUPP applies: read the mail
+content, interpret the prose instructions, and execute them. This enables ad-hoc
+tasks without creating formal beads.
+
+**Witness use case**: The Mayor or Deacon can send you mail with special instructions
+(e.g., "investigate polecat X which may be stuck"), then hook it. Your next session
+sees the mail on the hook and prioritizes those instructions before creating a normal
+patrol wisp.
+
 ---
 
 ## 📋 FOLLOWING YOUR MOL