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>

test/helpers/e2e-helpers.ts

@@ -5,6 +5,7 @@
  * tests across multiple files by category.
  */
 
+import '../../lib/conductor-env-shim';
 import { describe, test, beforeAll, afterAll, expect } from 'bun:test';
 import type { SkillTestResult } from './session-runner';
 import { EvalCollector, judgePassed } from './eval-store';