fix(costs): detect tmux session without TMUX env var

The TMUX environment variable is not inherited when Claude Code runs
bash commands, even though we are inside a tmux session. This caused
the Stop hook's 'gt costs record' to fail with:
  Error: --session flag required

Fix: Remove the early return that checked TMUX env var. The
tmux display-message command will naturally fail if we're not
in tmux, so the check was unnecessary and harmful.

Fixes: hq-to0lr

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

internal/cmd/costs.go

@@ -664,12 +664,9 @@ func deriveSessionName() string {
 
 // detectCurrentTmuxSession returns the current tmux session name if running inside tmux.
 // Uses `tmux display-message -p '#S'` which prints the session name.
+// Note: We don't check TMUX env var because it may not be inherited when Claude Code
+// runs bash commands, even though we are inside a tmux session.
 func detectCurrentTmuxSession() string {
-	// Check if we're inside tmux
-	if os.Getenv("TMUX") == "" {
-		return ""
-	}
-
 	cmd := exec.Command("tmux", "display-message", "-p", "#S")
 	output, err := cmd.Output()
 	if err != nil {