[plan] Enhance pick_experiment.cjs step summary with description and issue link

[github-actions]

Objective

Extend the GitHub Actions step summary produced by pick_experiment.cjs to include the experiment description and a link to the tracking issue when the enhanced frontmatter schema (from the companion issue) provides those fields.

Context

Issue #29604 (Area 3 — Step summary enhancements). After the schema enhancement sub-issue lands, pick_experiment.cjs will have access to description and issue fields per experiment. The step summary should surface these so engineers can understand the experiment purpose without reading the workflow YAML.

Approach

1. In actions/setup/js/pick_experiment.cjs, after reading experiment config, check whether config.description or config.issue are present.
2. Append to the Markdown step summary:
   • A Description row/section: > {description}
   • A Tracking issue link: [#{issue}](https://github.com/{owner}/{repo}/issues/{issue}) — use the GITHUB_REPOSITORY env var for the repo URL.
3. Only emit these extras when the fields are non-empty (backward-compatible with bare-array experiments that have no metadata).
4. Run make fmt-cjs && make lint-cjs to validate JavaScript.

Files to Modify

• actions/setup/js/pick_experiment.cjs — extend step summary generation

Acceptance Criteria

• Step summary includes description when description: field is set
• Step summary includes a GitHub issue link when issue: field is set
• No output change for experiments using the legacy bare-array form
• make lint-cjs passes
  Related to [ab-advisor] Improve experiment infrastructure: schema, reporting & audit #29604

Generated by Plan Command for issue #29604 · ● 253.3K · ◷

• expires on May 3, 2026, 8:12 PM UTC