feat(costs): Wire up gt costs record to Stop hook

Add Claude Code Stop hook that automatically records session costs when
sessions end. The hook calls `gt costs record` which now can derive the
session name from GT_* environment variables (GT_RIG, GT_POLECAT, GT_CREW,
GT_ROLE).

Changes:
- Add deriveSessionName() to infer tmux session name from environment
- Add Stop hook to settings-autonomous.json and settings-interactive.json
- Add unit tests for deriveSessionName function

When a Gas Town session ends, the Stop hook fires and records the session
cost as a bead event. Costs then appear in `gt costs --today` output.

Closes: gt-ntzhc

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

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

internal/cmd/costs.go

@@ -519,11 +519,14 @@ func runCostsRecord(cmd *cobra.Command, args []string) error {
 	// Get session from flag or try to detect from environment
 	session := recordSession
 	if session == "" {
-		// Try to get from TMUX_PANE or tmux environment
 		session = os.Getenv("GT_SESSION")
 	}
 	if session == "" {
-		return fmt.Errorf("--session flag required (or set GT_SESSION env var)")
+		// Derive session name from GT_* environment variables
+		session = deriveSessionName()
+	}
+	if session == "" {
+		return fmt.Errorf("--session flag required (or set GT_SESSION env var, or GT_RIG/GT_ROLE)")
 	}
 
 	t := tmux.NewTmux()
@@ -610,6 +613,41 @@ func runCostsRecord(cmd *cobra.Command, args []string) error {
 	return nil
 }
 
+// deriveSessionName derives the tmux session name from GT_* environment variables.
+// Session naming patterns:
+//   - Polecats: gt-{rig}-{polecat} (e.g., gt-gastown-toast)
+//   - Crew: gt-{rig}-crew-{crew} (e.g., gt-gastown-crew-max)
+//   - Witness/Refinery: gt-{rig}-{role} (e.g., gt-gastown-witness)
+//   - Mayor/Deacon: gt-{role} (e.g., gt-mayor)
+func deriveSessionName() string {
+	role := os.Getenv("GT_ROLE")
+	rig := os.Getenv("GT_RIG")
+	polecat := os.Getenv("GT_POLECAT")
+	crew := os.Getenv("GT_CREW")
+
+	// Polecat: gt-{rig}-{polecat}
+	if polecat != "" && rig != "" {
+		return fmt.Sprintf("gt-%s-%s", rig, polecat)
+	}
+
+	// Crew: gt-{rig}-crew-{crew}
+	if crew != "" && rig != "" {
+		return fmt.Sprintf("gt-%s-crew-%s", rig, crew)
+	}
+
+	// Global roles without rig: gt-{role}
+	if role != "" && rig == "" {
+		return fmt.Sprintf("gt-%s", role)
+	}
+
+	// Rig-based roles (witness, refinery): gt-{rig}-{role}
+	if role != "" && rig != "" {
+		return fmt.Sprintf("gt-%s-%s", rig, role)
+	}
+
+	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 {