Add human escalation path with severity levels (gt-1z3z)

Implements structured escalation channel for Gas Town:
- gt escalate command with CRITICAL/HIGH/MEDIUM severity levels
- Mayor startup check for pending escalations
- Escalation beads with tag for audit trail
- Mail routing to overseer with priority mapping
- Documentation in docs/escalation.md

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

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

docs/reference.md

@@ -222,6 +222,16 @@ gt mail send <addr> -s "Subject" -m "Body"
 gt mail send --human -s "..."    # To overseer
 ```
 
+### Escalation
+```bash
+gt escalate "topic"              # Default: MEDIUM severity
+gt escalate -s CRITICAL "msg"    # Urgent, immediate attention
+gt escalate -s HIGH "msg"        # Important blocker
+gt escalate -s MEDIUM "msg" -m "Details..."
+```
+
+See [escalation.md](escalation.md) for full protocol.
+
 ### Sessions
 ```bash
 gt handoff                   # Request cycle (context-aware)