fix(costs): derive session name for mayor/deacon without GT_TOWN

Error: Ran 1 stop hook
  ⎿  Stop hook error: Failed with non-blocking status code: Error: --session flag required (or set GT_SESSION env var, or GT_RIG/GT_ROLE)
  Usage:
    gt costs record [flags]

deriveSessionName() now falls back to gt-{role} when GT_ROLE is mayor
or deacon but GT_TOWN is not set. Previously this case returned empty
string, causing the Stop hook to fail.

internal/cmd/costs_test.go

@@ -61,6 +61,20 @@ func TestDeriveSessionName(t *testing.T) {
 			},
 			expected: "gt-ai-deacon",
 		},
+		{
+			name: "mayor session without GT_TOWN",
+			envVars: map[string]string{
+				"GT_ROLE": "mayor",
+			},
+			expected: "gt-mayor",
+		},
+		{
+			name: "deacon session without GT_TOWN",
+			envVars: map[string]string{
+				"GT_ROLE": "deacon",
+			},
+			expected: "gt-deacon",
+		},
 		{
 			name:     "no env vars",
 			envVars:  map[string]string{},