refactor: extract pure promotedEnv() from conductor-env-shim

Single source of truth for GSTACK_* key promotion semantics. The ambient
promoteConductorEnv() becomes a wrapper; behavior-preserving. Needed by the
hermetic env builder which must not mutate process.env.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/conductor-env-shim.ts

@@ -7,12 +7,30 @@
  *
  * Import this for its side effect: `import "../lib/conductor-env-shim";`
  */
-export function promoteConductorEnv(): void {
-  for (const key of ["ANTHROPIC_API_KEY", "OPENAI_API_KEY"] as const) {
-    if (!process.env[key] && process.env[`GSTACK_${key}`]) {
-      process.env[key] = process.env[`GSTACK_${key}`];
+const PROMOTED_KEYS = ["ANTHROPIC_API_KEY", "OPENAI_API_KEY"] as const;
+
+/**
+ * Pure form: returns a copy of `base` with each GSTACK_-prefixed key promoted
+ * to its canonical name when the canonical is empty. Single source of truth
+ * for promotion semantics — used by the ambient mutator below and by the
+ * hermetic env builder (test/helpers/hermetic-env.ts), which must not mutate
+ * process.env.
+ */
+export function promotedEnv(base: NodeJS.ProcessEnv): NodeJS.ProcessEnv {
+  const out: NodeJS.ProcessEnv = { ...base };
+  for (const key of PROMOTED_KEYS) {
+    if (!out[key] && out[`GSTACK_${key}`]) {
+      out[key] = out[`GSTACK_${key}`];
     }
   }
+  return out;
+}
+
+export function promoteConductorEnv(): void {
+  const promoted = promotedEnv(process.env);
+  for (const key of PROMOTED_KEYS) {
+    if (promoted[key]) process.env[key] = promoted[key];
+  }
 }
 
 promoteConductorEnv();