improvement(ci): check out same-repo pull requests through a git mirror

[waleedlatif1]

Summary

• Since ci: faster, cheaper, consistently named CI #8750/ci: duration-balanced integration shards, warm desktop-live, CI on every PR #8763 the checks run as ~19 parallel jobs, and a run finishes when its slowest job does. Checkout is now the long tail: median 11s, but 1 in 10 took a minute or more (up to 4 min), so one slow clone set the run's pace (a balanced 5.2 min integration shard took 9.8 min after a 4 min checkout)
• Same-repo pull requests now check out with useblacksmith/checkout (pinned v1.8.1). It keeps a git mirror on a sticky disk and fetches only what is new; on a non-Blacksmith runner, or if the mirror is unavailable, it falls back to a normal clone
• Fork pull requests and pushes keep actions/checkout. The mirror is one disk per repository that job steps can write to, and Git doesn't re-hash objects read through alternates, so it's shared only by runs that already share the pull_request caches. The deploy-gating push checks never read it
• Only checks.yml changes. Release, signing, publish and image jobs keep actions/checkout. Each job's mirror/plain pair is written once per fetch depth with YAML anchors

Type of Change

• Improvement

Testing

• Parsed the workflow and checked every job: each resolves to exactly one checkout for a same-repo PR, a fork PR, and a push, with its fetch depth unchanged (desktop-changes still checks out only on pull requests)
• actionlint 1.7.12, bun run lint, block registry check, check:audits (58), type-check, docs-manifest:check, root bun run test
• This PR's own CI run is the first mirror hydration, so the first warm run measures the checkout gain

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing (new tests pass the test-audit authoring gate)
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

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Project	Deployment	Actions	Updated
docs	Error		Oct 7, 2026 10:35pm UTC

[cubic-dev-ai]

All reported issues were addressed across 1 file

<sub>Reply with feedback, questions, or to request a fix.

Turn on auto-fix | Re-trigger cubic</sub>

[greptile-apps]

Confidence Score: 5/5

[High risk] Introduces a new third-party checkout action into CI.

The PR appears safe to merge; no actionable defects were found.

What we checked:

• Forks and pushes avoid mirror: The mirror step requires a PR from a non-fork repository. The plain step covers forks and non-PR events.
• Desktop checkout remains PR-only: Each PR selects one checkout with fetch-depth: 2. Non-PR events skip both checkouts and the diff step, as before.

Summary

Same-repository PR jobs now select pinned useblacksmith/checkout to reuse a Git mirror.

• Fork PRs and push runs retain actions/checkout.
• YAML anchors share the checkout pairs while preserving the existing fetch-depth settings.
• No actionable issues were found.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[Checks job] --> B{Pull request?}
  B -->|No| C[Plain checkout]
  B -->|Yes| D{Fork repository?}
  D -->|Yes| C
  D -->|No| E[Mirror checkout]
  C --> F[Existing checks]
  E --> F

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