benchcheck: add job-urls subcommand to replace inline gh api --jq

[michelle-clayton-work]

Move the benchmark workflow's job-URL lookup out of an inline
gh api ... --jq pipeline and into a tested benchcheck job-urls
subcommand.

What changed

• New benchcheck job-urls [jobs.json] — reads the GitHub "list jobs
  for a workflow run" API response (from a file or stdin), and writes a TSV
  mapping each bench (<label>) matrix job to its job URL, deep-linked to the
  compare step when present. It handles the concatenated JSON pages that
  gh api --paginate produces. This is the same output the previous --jq
  program produced, and is consumed by benchcheck report -job-urls.

• benchmark.yml — the conclude job now builds benchcheck once, then
  runs gh api ... --paginate > jobs.json (gh still handles auth and
  pagination) piped through benchcheck job-urls, replacing the inline
  --jq filter. The prebuilt binary is reused by the report step.

Why

The label/step-link extraction was an inline jq program embedded in YAML —
hard to read, hard to test, and easy to break. Moving it into benchcheck
makes it unit-testable and keeps the workflow step simple.

Testing

gofmt, go vet, and go test ./cmd/benchcheck/ all clean. New
jobs_test.go covers deep-linking to the compare step, a skipped compare
step (no deep link), ignoring non-benchmark jobs, multi-page (--paginate)
input, and empty input. Also smoke-tested the built binary against a sample
jobs response via stdin.

• https://github.com/microsoft/go-lab/issues/342

[Copilot]

Pull request overview

This PR refactors the benchmark workflow’s “job URL lookup” logic by moving it out of an inline gh api ... --jq pipeline in the GitHub Actions workflow and into a dedicated, unit-tested benchcheck job-urls subcommand. This keeps the workflow simpler and makes the label/URL extraction logic testable and easier to maintain within the Go codebase.

Changes:

• Added a new benchcheck job-urls [jobs.json] subcommand that parses one or more concatenated GitHub “list jobs for a workflow run” JSON pages and emits <label>\t<url> TSV lines, deep-linking to the first non-skipped “compare” step when present.
• Added unit tests covering compare-step deep linking, skipped compare steps, ignoring non-benchmark jobs, multi-page paginated input, and empty jobs lists.
• Updated .github/workflows/benchmark.yml to build benchcheck once and use benchcheck job-urls instead of an inline --jq program.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
cmd/benchcheck/main.go	Registers the new job-urls subcommand and documents it in the command list.
cmd/benchcheck/jobs.go	Implements benchcheck job-urls parsing and TSV generation, including compare-step deep links and paginated input handling.
cmd/benchcheck/jobs_test.go	Adds unit coverage for the new jobs parsing and URL deep-link behavior.
.github/workflows/benchmark.yml	Simplifies the workflow by replacing inline jq with the tested benchcheck job-urls command and reusing a single built binary.

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