fix(ci): make merge queue lane build-only in build.yml

[castrojo]

Root cause

The merge queue has silently ejected every entry since 2026-07-06 (PRs #802, #815, #817, #824, #825 — nothing has merged in two weeks). common#660 added if: github.event_name != 'merge_group' to the Export image for scanning and Scan image for CVEs steps, but left these on != 'pull_request' (which matches merge_group):

• Promote image to root storage — reads /tmp/scan-image.tar, which the skipped export never produced: open /tmp/scan-image.tar: no such file or directory (failing merge-group run)
• Push image / Write digest / Upload digest / manifest job — these would push per-arch tags and re-sign :latest from a throwaway queue ref if the promote step hadn't failed first

Fix

Merge queue lane is now build-only: every push-lane step carries github.event_name != 'pull_request' && github.event_name != 'merge_group'. The queue verifies the combined commit builds; pushing, digests, manifests, and signing stay exclusive to the push/dispatch lanes.

Self-healing: merge_group runs execute the workflow from the merged ref, so this PR's own queue run uses the fixed YAML.

Skill update (same PR)

docs/skills/ci-tooling.md merge_group section extended with the build-only rule and the artifact-lane tracing checklist.

Validation

• actionlint + full pre-commit: pass
• just check: pass

Unblocks #824 (MIME defaults) and #825 (game-devices-udev checksum), plus the stalled Renovate queue.

Summary by CodeRabbit

• Bug Fixes

  • Prevented merge queue builds from attempting image promotion, publishing, manifest creation, or digest uploads when required artifacts are unavailable.

• Documentation

  • Added guidance for correctly gating merge queue workflow steps and tracing artifact availability across build lanes.

[coderabbitai]

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: c7f832ee-36fd-423a-8ad3-01b207c99517

📥 Commits

Reviewing files that changed from the base of the PR and between 8c4a4a8 and 9d592bd.

📒 Files selected for processing (2)

• .github/workflows/build.yml
• docs/skills/ci-tooling.md

📝 Walkthrough

Walkthrough

The build workflow now skips image promotion, pushing, digest handling, and manifest creation for merge_group events. CI tooling documentation describes merge queue builds as build-only and requires matching guards for artifact-consuming or publishing steps.

Changes

Merge queue gating

Layer / File(s)	Summary
Build-only merge queue lane .github/workflows/build.yml, docs/skills/ci-tooling.md	Workflow publishing and digest steps exclude pull_request and merge_group events; documentation records the related artifact and publishing guard rule.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• projectbluefin/common#660: Updates the same workflow’s event gating to prevent merge queue artifact failures.

Suggested labels: kind/automation, kind/documentation, size:XS

Suggested reviewers: rep​ires, kikaraage

Poem

I’m a rabbit guarding the queue,
No publish steps sneak through.
Builds hop, artifacts stay sound,
Merge lanes leave no scraps around.
Thump-thump—green checks bloom!

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The PR description explains the fix, but it does not follow the template: it lacks the required 'What does this change?' and 'Why?'/Closes # sections.	Add the required template sections with a one-sentence change summary, a 'Closes #NNN' rationale, and the checklist items from the repository template.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise, specific, and matches the main CI change to make the merge queue lane build-only.
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 fix/ci/merge-group-build-only

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