feat(witness): Implement Witness as Claude agent (gt-xicq)

Convert the Witness from a Go polling loop to a Claude agent with:

- Enhanced witness.md.tmpl with detailed agent instructions:
  - Startup protocol for checking handoffs and polecat status
  - Lifecycle request processing (verify git state, cleanup)
  - Health check protocol for detecting stuck polecats
  - Nudge protocol with 3-strike escalation
  - Escalation protocol for unresolvable issues
  - Session cycling with handoff mail

- Updated gt witness attach to:
  - Create sessions in <rig>/witness/ directory for proper role detection
  - Ensure witness directory exists

- Added startup directive for Witness role in gt prime

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

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

internal/cmd/prime.go

@@ -401,6 +401,16 @@ func outputStartupDirective(ctx RoleContext) {
 		fmt.Println("2. Check mail: `gt mail inbox`")
 		fmt.Println("3. If there's a 🤝 HANDOFF message, read it and summarize")
 		fmt.Println("4. If no mail, await user instruction")
+	case RoleWitness:
+		fmt.Println()
+		fmt.Println("---")
+		fmt.Println()
+		fmt.Println("**STARTUP PROTOCOL**: You are the Witness. Please:")
+		fmt.Println("1. Check for handoff: `gt mail inbox` - look for 🤝 HANDOFF messages")
+		fmt.Println("2. Check polecat status: `gt polecat list " + ctx.Rig + " --json`")
+		fmt.Println("3. Process any lifecycle requests from inbox")
+		fmt.Println("4. If polecats stuck/idle, nudge them")
+		fmt.Println("5. If all quiet, wait for activity")
 	case RolePolecat:
 		fmt.Println()
 		fmt.Println("---")