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/witness.go

@@ -254,11 +254,17 @@ func runWitnessAttach(cmd *cobra.Command, args []string) error {
 		return fmt.Errorf("checking session: %w", err)
 	}
 
+	// Witness working directory - use <rig>/witness/ for proper role detection
+	witnessDir := filepath.Join(r.Path, "witness")
+	if err := os.MkdirAll(witnessDir, 0755); err != nil {
+		return fmt.Errorf("creating witness directory: %w", err)
+	}
+
 	if !running {
 		// Start witness session (like Mayor)
 		fmt.Printf("Starting witness session for %s...\n", rigName)
 
-		if err := t.NewSession(sessionName, r.Path); err != nil {
+		if err := t.NewSession(sessionName, witnessDir); err != nil {
 			return fmt.Errorf("creating session: %w", err)
 		}