fix(costs): auto-detect tmux session in Stop hook

gt costs record now auto-detects the current tmux session when
running inside tmux, so the Stop hook no longer requires --session
flag or GT_SESSION env var.

Detection order:
1. --session flag
2. GT_SESSION env var
3. GT_RIG/GT_ROLE derivation
4. Current tmux session (new)

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

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

internal/cmd/costs.go

@@ -525,6 +525,10 @@ func runCostsRecord(cmd *cobra.Command, args []string) error {
 		// Derive session name from GT_* environment variables
 		session = deriveSessionName()
 	}
+	if session == "" {
+		// Try to detect current tmux session (works when running inside tmux)
+		session = detectCurrentTmuxSession()
+	}
 	if session == "" {
 		return fmt.Errorf("--session flag required (or set GT_SESSION env var, or GT_RIG/GT_ROLE)")
 	}
@@ -649,6 +653,28 @@ func deriveSessionName() string {
 	return ""
 }
 
+// detectCurrentTmuxSession returns the current tmux session name if running inside tmux.
+// Uses `tmux display-message -p '#S'` which prints the session name.
+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 {
+		return ""
+	}
+
+	session := strings.TrimSpace(string(output))
+	// Only return if it looks like a Gas Town session
+	if strings.HasPrefix(session, constants.SessionPrefix) {
+		return session
+	}
+	return ""
+}
+
 // buildAgentPath builds the agent path from role, rig, and worker.
 // Examples: "mayor", "gastown/witness", "gastown/polecats/toast"
 func buildAgentPath(role, rig, worker string) string {