Smart attach: link window when inside tmux

When 'gt X attach' is run from inside a tmux session, link the target
session's window as a new tab instead of switching sessions entirely.
Use C-b n/p to navigate between tabs.

Outside tmux: unchanged behavior (full attach)
Inside tmux: links window as tab for convenient multi-session viewing

- Add tmux.LinkWindow() and tmux.IsInsideTmux()
- Update attachToTmuxSession() with smart detection
- Update mayor, deacon, crew, refinery attach commands

internal/tmux/tmux.go

@@ -5,6 +5,7 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
+	"os"
 	"os/exec"
 	"strings"
 	"time"
@@ -519,6 +520,21 @@ func (t *Tmux) ConfigureGasTownSession(session string, theme Theme, rig, worker,
 	return nil
 }
 
+// LinkWindow links a window from another session into the current session.
+// This allows viewing another session's window as a tab without switching sessions.
+// Useful when already inside tmux and want to see another session.
+func (t *Tmux) LinkWindow(sourceSession string, windowIndex int) error {
+	source := fmt.Sprintf("%s:%d", sourceSession, windowIndex)
+	_, err := t.run("link-window", "-s", source)
+	return err
+}
+
+// IsInsideTmux checks if the current process is running inside a tmux session.
+// This is detected by the presence of the TMUX environment variable.
+func IsInsideTmux() bool {
+	return os.Getenv("TMUX") != ""
+}
+
 // SetMailClickBinding configures left-click on status-right to show mail preview.
 // This creates a popup showing the first unread message when clicking the mail icon area.
 func (t *Tmux) SetMailClickBinding(session string) error {