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>

bin/gstack-gbrain-sync.ts

@@ -35,6 +35,7 @@ import { execSync, spawnSync } from "child_process";
 import { homedir } from "os";
 import { createHash } from "crypto";
 
+import "../lib/conductor-env-shim";
 import { detectEngineTier, withErrorContext, canonicalizeRemote } from "../lib/gstack-memory-helpers";
 import { ensureSourceRegistered, sourcePageCount } from "../lib/gbrain-sources";
 import { localEngineStatus, type LocalEngineStatus } from "../lib/gbrain-local-status";

bin/gstack-model-benchmark

@@ -24,6 +24,7 @@
  *   gstack-model-benchmark --prompt "hi" --models claude,gpt,gemini --dry-run
  */
 
+import '../lib/conductor-env-shim';
 import * as fs from 'fs';
 import * as path from 'path';
 import { runBenchmark, formatTable, formatJson, formatMarkdown, type BenchmarkInput } from '../test/helpers/benchmark-runner';