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>

scripts/preflight-agent-sdk.ts

@@ -16,6 +16,7 @@
  * side effects beyond stdout and a ~15 token API call.
  */
 
+import '../lib/conductor-env-shim';
 import { query, type SDKMessage } from '@anthropic-ai/claude-agent-sdk';
 import { readOverlay } from './resolvers/model-overlay';
 import { resolveClaudeBinary } from '../browse/src/claude-bin';