test(benchmark): add structured benchmark report output

[jafreck]

Summary

Adds structured JSON benchmark report output for the Copilot benchmark harness and ignores the local autoresearch/ directory.

Changes

• write a structured JSON report to benchmark-results/<repo>.json from tests/benchmark/copilot-agent.test.ts
• add StructuredTaskResult and StructuredBenchmarkReport helpers in tests/benchmark/util/scorer.ts
• include per-task deltas, warnings, aggregate comparison, and optional significance data in the structured output
• ignore autoresearch/ in .gitignore

Validation

• npx vitest run tests/benchmark/copilot-agent.test.ts
  • file loaded successfully
  • all tests remained skipped as expected without BENCHMARK_COPILOT=1

Scope

This PR is intentionally limited to:

• .gitignore
• tests/benchmark/copilot-agent.test.ts
• tests/benchmark/util/scorer.ts

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.01%. Comparing base (942ef41) to head (fcd540d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #223   +/-   ##
=======================================
  Coverage   88.01%   88.01%           
=======================================
  Files          85       85           
  Lines        9545     9545           
  Branches     2951     2951           
=======================================
  Hits         8401     8401           
  Misses       1144     1144           

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