feat: back model benchmark with Braintrust, drop in-house scoring

Braintrust now owns benchmark scoring, experiments, comparison, and
reporting. GStack keeps only the CLI-agent adapters (the unavoidable shim)
plus operational metrics the CLIs report. runProviderBenchmark wraps each
adapter as a Braintrust Eval task; a deterministic required-terms scorer
replaces the in-house evaluation logic and the optional autoevals ClosedQA
judge replaces judge.ts.

Runs local by default under bun: with no BRAINTRUST_API_KEY it sets
noSendLogs and ships nothing; setting the key opts into the cloud dashboard.
An empty output with zero tokens is thrown so a silent auth/CLI failure
can't masquerade as a 0.0 score.

Deletes runner.ts and judge.ts (and their test-helper re-export shims);
rewires bin/gstack-model-benchmark and adapts the benchmark tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/benchmark-production-boundary.test.ts

@@ -42,12 +42,8 @@ test('production modules do not import from test directories', () => {
 
 test('former test-helper paths re-export the production benchmark API', async () => {
   const [
-    runner,
-    helperRunner,
     pricing,
     helperPricing,
-    judge,
-    helperJudge,
     claude,
     helperClaude,
     gpt,
@@ -55,12 +51,8 @@ test('former test-helper paths re-export the production benchmark API', async ()
     gemini,
     helperGemini,
   ] = await Promise.all([
-    import('../lib/model-benchmark/runner'),
-    import('./helpers/benchmark-runner'),
     import('../lib/model-benchmark/pricing'),
     import('./helpers/pricing'),
-    import('../lib/model-benchmark/judge'),
-    import('./helpers/benchmark-judge'),
     import('../lib/model-benchmark/providers/claude'),
     import('./helpers/providers/claude'),
     import('../lib/model-benchmark/providers/gpt'),
@@ -69,9 +61,7 @@ test('former test-helper paths re-export the production benchmark API', async ()
     import('./helpers/providers/gemini'),
   ]);
 
-  expect(helperRunner.runBenchmark).toBe(runner.runBenchmark);
   expect(helperPricing.estimateCostUsd).toBe(pricing.estimateCostUsd);
-  expect(helperJudge.judgeEntries).toBe(judge.judgeEntries);
   expect(helperClaude.ClaudeAdapter).toBe(claude.ClaudeAdapter);
   expect(helperGpt.GptAdapter).toBe(gpt.GptAdapter);
   expect(helperGemini.GeminiAdapter).toBe(gemini.GeminiAdapter);