docs(ci): add runner-capacity diagnosis for slow pipelines

[CybotTM]

What

New reference references/ci-runner-capacity.md (indexed from SKILL.md): how to diagnose a slow GitHub Actions pipeline before optimizing it.

The lesson, from a real CI-speedup engagement: after cutting a repo's functional-test compute by 57% and each cell ~4×, PR wall clock did not move — because the pipeline was runner-acquisition-bound, not job-duration-bound. Peak concurrency was 11–15 against a cap of 20, so jobs were waiting individually for machines, not on each other; trimming job count would have bought nothing.

The reference documents:

• the org-wide concurrency cap by plan (Free 20 / Team 60 / Enterprise 500), independent of repo visibility;
• the two regimes (cap-bound vs acquisition-bound) and why the distinction decides the fix;
• how to measure — Σ queue job-min vs Σ run job-min, and a sweep-line peak concurrency — with a ready gh api snippet;
• that under the acquisition-bound regime fan-out is correct and the real lever is capacity (plan upgrade / self-hosted, with the public-repo untrusted-code caveat);
• that cutting compute still helps (lowers org-wide queue pressure) even when it buys no wall clock;
• controlling for run-to-run queue noise (compare runs at similar queue pressure, not two arbitrary runs).

Notes

• New reference file + one SKILL.md index row. Body word count 436 (well under the cap). No version bump (release flow owns that).
• markdownlint-cli2: 0 errors. Signed + DCO.

Came from /retro: yes

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[gemini-code-assist]

Code Review

This pull request adds a new reference document, ci-runner-capacity.md, which provides guidance on diagnosing slow GitHub Actions pipelines by distinguishing between cap-bound and acquisition-bound regimes, and registers it in SKILL.md. Feedback was provided to replace angle-bracket placeholders (e.g., <run-id>) in the bash code block with plain text placeholders to avoid shell redirection syntax errors.

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[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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