crew at: don't link windows by default, add --detached flag

When running `gt crew at` from inside tmux:
- No longer auto-links the target session as a tab
- Just prints "Started X. Use C-b s to switch."
- User stays in their current pane

When running from outside tmux:
- Default: attach to the session (existing behavior)
- With -d/--detached: start session without attaching

This gives users more control over tmux session navigation and
reduces confusion for tmux newcomers.

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

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

internal/cmd/crew_at.go

@@ -160,6 +160,18 @@ func runCrewAt(cmd *cobra.Command, args []string) error {
 		return execClaude("gt prime")
 	}
 
-	// Attach to session using exec to properly forward TTY
+	// If inside tmux (but different session), don't switch - just inform user
+	if tmux.IsInsideTmux() {
+		fmt.Printf("Started %s/%s. Use C-b s to switch.\n", r.Name, name)
+		return nil
+	}
+
+	// Outside tmux: attach unless --detached flag is set
+	if crewDetached {
+		fmt.Printf("Started %s/%s. Run 'gt crew at %s' to attach.\n", r.Name, name, name)
+		return nil
+	}
+
+	// Attach to session
 	return attachToTmuxSession(sessionID)
 }