feat(polecat): Add gt polecat status command (gt-6lt3)

Implements detailed polecat status display including:
- Lifecycle state (working, done, stuck, idle)
- Assigned issue
- Session status (running/stopped, attached/detached)
- Session creation time
- Last activity time with relative "ago" format

Also extends tmux.SessionInfo and session.Info to include
last activity timestamp from tmux session_activity.

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

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

internal/session/manager.go

@@ -69,6 +69,9 @@ type Info struct {
 
 	// Windows is the number of tmux windows.
 	Windows int `json:"windows,omitempty"`
+
+	// LastActivity is when the session last had activity.
+	LastActivity time.Time `json:"last_activity,omitempty"`
 }
 
 // sessionName generates the tmux session name for a polecat.
@@ -254,6 +257,14 @@ func (m *Manager) Status(polecat string) (*Info, error) {
 		}
 	}
 
+	// Parse activity time (unix timestamp from tmux)
+	if tmuxInfo.Activity != "" {
+		var activityUnix int64
+		if _, err := fmt.Sscanf(tmuxInfo.Activity, "%d", &activityUnix); err == nil && activityUnix > 0 {
+			info.LastActivity = time.Unix(activityUnix, 0)
+		}
+	}
+
 	return info, nil
 }