ci(setup-minimal): route package-cache reads through the R2 mirror

[bryan-minimal]

Replaces #613 — that PR accidentally included unrelated working-tree files (my git add -A in a live checkout; the WIP has been restored to its owner untouched). This one is the intended single-file change.

One step added to the shared setup-minimal action, mirroring webapp#380 — with the bill quantified.

Why: the six per-PR advisory workflows (smoke-test-coverage, corresponding-source, stable-closure-check, trivial-update, license-lint, version-age) each run on a fresh GitHub-hosted Azure runner and cold-pull ~6.5 GB of toolchain artifacts from GCS in 90 seconds. Across the fleet that's ~370 GB/week from ~174 runner IPs — the majority of remaining billed GCS egress ($250 CAD/mo at current pace). Attribution from GCS usage logs, timestamp-matched to this repo's workflow launch batches (runner IP active 21:22–21:24 ↔ workflows launched 21:22:13).

Why it's safe: cache.minimal.farm serves the identical immutable objects from R2 (verified byte-identical against authoritative GCS); the CLI sha256-verifies every artifact regardless of source; MINIMAL_REMOTE_CACHE_URL is honored by every build on the unstable channel (minimal#643) and continuously canaried by minimal's dogfood lane (minimal#667). webapp has run this way since July 8. Rollback = delete the step.

(Uses .farm for now — it flips to cache.minimal.dev for free when minimal#1234 makes that the client default, at which point this step becomes a harmless no-op that can be deleted.)

🤖 Generated with Claude Code

Summary by CodeRabbit

• Chores
  • Configured package-cache reads to use the designated remote cache mirror in minimal setup workflows.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 0beb0427-f8c9-485c-a893-60926b5b1ff1

📥 Commits

Reviewing files that changed from the base of the PR and between 9cf7e80 and da61b1b.

📒 Files selected for processing (1)

• .github/actions/setup-minimal/action.yml

Included review availability: 1 review is currently available. Based on recent review activity, included reviews refill at 2 per hour.

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📝 Walkthrough

Walkthrough

The minimal setup action now exports the R2 mirror URL through GITHUB_ENV. Later job steps can use cache.minimal.farm for package-cache reads.

Changes

Package cache mirror

Layer / File(s)	Summary
Export the package cache mirror URL .github/actions/setup-minimal/action.yml	Adds a shell step that writes the trailing-slash R2 mirror URL to GITHUB_ENV for subsequent steps.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: ⚪ Minimal · up to da61b

This localized CI configuration change reroutes package-cache reads through the mirror; no actionable merge-blocking risk remains beyond normal checks and review.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: routing package-cache reads through the R2 mirror.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/r2-mirror-reads

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