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_test.go

@@ -0,0 +1,98 @@
+package cmd
+
+import (
+	"os"
+	"testing"
+)
+
+func TestDeriveSessionName(t *testing.T) {
+	tests := []struct {
+		name     string
+		envVars  map[string]string
+		expected string
+	}{
+		{
+			name: "polecat session",
+			envVars: map[string]string{
+				"GT_ROLE":    "polecat",
+				"GT_RIG":     "gastown",
+				"GT_POLECAT": "toast",
+			},
+			expected: "gt-gastown-toast",
+		},
+		{
+			name: "crew session",
+			envVars: map[string]string{
+				"GT_ROLE": "crew",
+				"GT_RIG":  "gastown",
+				"GT_CREW": "max",
+			},
+			expected: "gt-gastown-crew-max",
+		},
+		{
+			name: "witness session",
+			envVars: map[string]string{
+				"GT_ROLE": "witness",
+				"GT_RIG":  "gastown",
+			},
+			expected: "gt-gastown-witness",
+		},
+		{
+			name: "refinery session",
+			envVars: map[string]string{
+				"GT_ROLE": "refinery",
+				"GT_RIG":  "gastown",
+			},
+			expected: "gt-gastown-refinery",
+		},
+		{
+			name: "mayor session",
+			envVars: map[string]string{
+				"GT_ROLE": "mayor",
+			},
+			expected: "gt-mayor",
+		},
+		{
+			name: "deacon session",
+			envVars: map[string]string{
+				"GT_ROLE": "deacon",
+			},
+			expected: "gt-deacon",
+		},
+		{
+			name:     "no env vars",
+			envVars:  map[string]string{},
+			expected: "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Save and clear relevant env vars
+			saved := make(map[string]string)
+			envKeys := []string{"GT_ROLE", "GT_RIG", "GT_POLECAT", "GT_CREW"}
+			for _, key := range envKeys {
+				saved[key] = os.Getenv(key)
+				os.Unsetenv(key)
+			}
+			defer func() {
+				// Restore env vars
+				for key, val := range saved {
+					if val != "" {
+						os.Setenv(key, val)
+					}
+				}
+			}()
+
+			// Set test env vars
+			for key, val := range tt.envVars {
+				os.Setenv(key, val)
+			}
+
+			result := deriveSessionName()
+			if result != tt.expected {
+				t.Errorf("deriveSessionName() = %q, want %q", result, tt.expected)
+			}
+		})
+	}
+}