docs(ci-runners): correct the runner table and document the lane variables

[teamleaderleo]

What

Documentation only. Corrects docs/ci-runners.md, which tests/test_ci_self_hosted_guard.sh points people at as the reference for which runner a job lands on.

Three things were wrong

1. The "Active value" column had drifted. It asserted values for five variables that had not been set for days. A column claiming live state, in a file that cannot observe live state, will always rot — so it is now "Intended steady state", with a pointer to gh variable list --repo manaflow-ai/cmux for what is actually set.

The intended values are unchanged from what the file already documented, except MACOS_RUNNER_15 and MACOS_RUNNER_DISPLAY, which now name the Blacksmith pool rather than Tart labels. That matches the file's own "Tart isolation and capacity" section, which already says macOS 26 and release builds stay on Blacksmith until a Tahoe image passes the runner and GUI canaries.

2. Three variables were undocumented. MACOS_RUNNER_PR (#13658) and MACOS_RUNNER_TESTS (#13662) were added and never written down — my omission in both PRs. MACOS_RUNNER_STREAMED_VALIDATION has been set for days with no row at all, and its two consumers disagree on the fallback (ios-streamed-validate.yml falls back to macOS 26, iroh-release-gate.yml to macOS 15), which is now stated rather than left to be discovered.

A new Lanes section explains why macOS jobs do not all follow one variable:

• Required CI on main, the merge queue, releases and nightly follow the MACOS_RUNNER_* variables. This is where a queued runner blocks a merge or a ship.
• Pull requests resolve MACOS_RUNNER_PR first. They are cancelled on supersession by design, so they are the wrong place for elastic paid capacity.
• Manual test debugging resolves MACOS_RUNNER_TESTS and deliberately does not follow MACOS_RUNNER_15.

3. The Blacksmith cutover recipe pointed at a retired provider. It set MACOS_RUNNER_DISPLAY to depot-macos-latest. Depot is retired for cmux CI — test-depot.yml says so in its own comment — and depot-macos-latest is not in .github/actionlint.yaml's label allowlist. Running that recipe as written would point the GUI lane at a pool that no longer answers. It now names the same Blacksmith label as the workflow fallback.

Also

Records that fork pull requests resolve MACOS_RUNNER_PR and MACOS_RUNNER_TESTS to empty, so they land on the Blacksmith fallback — the same runner they used before those variables existed. That property is easy to forget and easy to break.

Test

tests/test_ci_self_hosted_guard.sh passes. No workflow or script changes.

Related

• CI cost: macOS runner variables are pointed at the paid Warp overflow; five edits move ~11.6k runner-min/day to Blacksmith #13652 — the measurement, and the variable changes still outstanding
• ci: route pull-request macOS jobs to Blacksmith, keep overflow for the rest #13658, ci: give the manual test-debugging lanes their own Blacksmith-default runner #13662 — the PRs that introduced the two lane variables

🤖 Generated with Claude Code

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Summary by cubic

Fixes the CI runner reference in docs/ci-runners.md — the doc tests/test_ci_self_hosted_guard.sh points people at — which had a stale "Active value" column, left three lane variables undocumented, and named a retired provider in the cutover recipe.

The column now reads "Intended steady state" and points readers at gh variable list for what is actually set; MACOS_RUNNER_15 and MACOS_RUNNER_DISPLAY now name the Blacksmith pool rather than Tart labels. Adds rows and a Lanes section for MACOS_RUNNER_PR, MACOS_RUNNER_TESTS, and MACOS_RUNNER_STREAMED_VALIDATION, explaining that required CI on main, the merge queue, releases and nightly follow the MACOS_RUNNER_* variables while pull requests and manual test lanes stay on Blacksmith by default. The cutover recipe set MACOS_RUNNER_DISPLAY to depot-macos-latest, a retired provider not in the actionlint label allowlist; it now uses the same Blacksmith label as the workflow fallback. Records that fork pull requests resolve the two new variables to empty and land on Blacksmith, the runner they used before. Documentation only; no workflow or script changes.

^{Written for commit 26fc399. Summary will update on new commits.}

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