fix: Add polecat handling to sessionToAgentID (gt-giyhp)

The sessionToAgentID function was missing handling for polecat sessions,
causing gt sling to fail to update the agent bead's hook_bead field.

Before: gt-gastown-nux -> gt-gastown-nux (unchanged, no match)
After:  gt-gastown-nux -> gastown/polecats/nux (correct format)

This enables updateAgentHookBead to properly convert to the agent bead ID
(gt-polecat-gastown-nux) and update the hook_bead field, which is
required for agents to know they have work on their hook.

Found via E2E test (gt-j0gx2) - nux wasn't picking up slung work because
hook_bead was null despite the task being pinned.

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

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

internal/cmd/sling.go

@@ -397,6 +397,14 @@ func sessionToAgentID(session string) string {
 		rig := strings.TrimPrefix(session, "gt-")
 		rig = strings.TrimSuffix(rig, "-refinery")
 		return fmt.Sprintf("%s/refinery", rig)
+	case strings.HasPrefix(session, "gt-"):
+		// gt-gastown-nux -> gastown/polecats/nux (polecat)
+		parts := strings.Split(strings.TrimPrefix(session, "gt-"), "-")
+		if len(parts) >= 2 {
+			rig := parts[0]
+			name := strings.Join(parts[1:], "-")
+			return fmt.Sprintf("%s/polecats/%s", rig, name)
+		}
 	}
 	return session
 }