feat: GSTACK_* env-key shim for Conductor workspaces

New lib/conductor-env-shim.ts promotes GSTACK_ANTHROPIC_API_KEY and
GSTACK_OPENAI_API_KEY to canonical names when canonical is empty. Wired
into the four TS entry points that hit paid APIs or gbrain embeddings:
gstack-gbrain-sync.ts, gstack-model-benchmark, preflight-agent-sdk.ts,
test/helpers/e2e-helpers.ts. Side-effect-only import, 15 lines total.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/conductor-env-shim.ts

@@ -0,0 +1,14 @@
+/**
+ * Conductor workspaces don't inherit the user's interactive shell env, so the
+ * canonical ANTHROPIC_API_KEY / OPENAI_API_KEY may be missing while
+ * Conductor's GSTACK_-prefixed forms are present. Promote the GSTACK_ form to
+ * canonical when canonical is empty, so subprocesses (gbrain embed,
+ * @anthropic-ai/claude-agent-sdk, etc) pick it up.
+ *
+ * Import this for its side effect: `import "../lib/conductor-env-shim";`
+ */
+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}`];
+  }
+}