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/helpers/benchmark-judge.ts

@@ -1,2 +0,0 @@
-// Compatibility export for tests and downstream tooling that used the former helper path.
-export * from '../../lib/model-benchmark/judge';

test/helpers/benchmark-runner.ts

@@ -1,2 +0,0 @@
-// Compatibility export for tests and downstream tooling that used the former helper path.
-export * from '../../lib/model-benchmark/runner';