ci: move mingw + clang-cl Windows builds to nightly

[borisbat]

What

Move the two heaviest, lowest-per-PR-signal lanes — build_windows_mingw and build_windows_clangcl (~26 min each, alternative Windows toolchains) — off per-PR CI and onto a nightly schedule.

In build.yml:

• Added schedule: (daily 08:00 UTC) + the jobs already had workflow_dispatch.
• build_windows_mingw / build_windows_clangcl: if: … && (github.event_name == 'schedule' || github.event_name == 'workflow_dispatch') → run only nightly or on manual dispatch.
• build matrix / bundle_smoke / build_linux_gcc: if: … && github.event_name != 'schedule' → stay per-PR/push, skip on the nightly schedule. So nightly runs only the two toolchains — no redundant nightly matrix.
• Concurrency key now includes github.event_name so a nightly run can't cancel an in-flight master-push run (or vice versa).

Net: every PR drops ~52 lane-minutes of Windows-toolchain build; a toolchain regression still gets caught within ~24 h (or immediately via gh workflow run build.yml --ref <branch>).

The updated PR/CI workflow (documented here)

Context for why this is safe + how to work now (full detail in skills/pr_review_iteration.md + skills/preflight.md):

• CI is free + auto-runs on every push (public-repo standard runners, since the free-runner switch in ci: switch all runners to standard free runners (temporary, unblock spending limit) #3233) → the PR badge gates honestly on the latest commit.
• Iterate against Copilot, not the matrix. Copilot review (review_on_push) returns in ~5 min and is free (ruleset check, not a runner). The full matrix is ~30 min. So: push → ~5-min Copilot → fix → repeat, without sitting through the 30-min matrix each round. Only once Copilot is dry, wait the ~30 min for CI, then merge on green.
• A force-push doesn't trigger review_on_push — prefer normal commits during iteration (squash-merge collapses them at the end); after a force-push, re-request Copilot manually.

Note

This is the small remaining trim after closing the explicit-trigger experiment (#3234) — that approach was both redundant (free runners already solved the cost) and un-gateable (workflow_dispatch runs aren't PR-associated, so the badge lied). This PR keeps the simple, honest, free model and just relocates the two long-poles to nightly.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Moves the two slowest Windows alternative-toolchain CI jobs (MinGW + clang-cl) off per-PR/push runs and onto a nightly schedule (plus manual dispatch), reducing per-PR CI time while retaining periodic coverage for toolchain regressions.

Changes:

• Add a daily schedule trigger and gate MinGW/clang-cl jobs to run only on schedule or workflow_dispatch.
• Gate the main build matrix, bundle smoke test, and Linux GCC warning gate to not run on schedule (so nightly runs only the toolchain jobs).
• Adjust workflow concurrency grouping to include github.event_name to avoid cross-event cancellations.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
skills/preflight.md	Updates CI lane documentation to reflect per-PR vs nightly split; clarifies how to run the nightly toolchain lanes.
skills/pr_review_iteration.md	Updates iteration guidance to reflect the new CI cadence and nightly-only toolchain coverage.
.github/workflows/build.yml	Adds nightly schedule and gates jobs so only the two heavy Windows toolchain lanes run nightly; updates concurrency key.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

.github/workflows/build.yml:627

• build_windows_mingw runs gh cache delete ... (see the "Refresh sccache slot" step later in this job), but the job does not request actions: write permissions. Other jobs that delete caches (e.g. the build matrix) explicitly set this permission; if the repo/workflow defaults are read-only, the nightly run will fail here.

Add job-level permissions mirroring the matrix job (contents: read, actions: write).

    if: github.event_name == 'schedule' || github.event_name == 'workflow_dispatch'
    runs-on: windows-latest
    defaults:
      run:
        shell: msys2 {0}

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.