ci: leave a merge receipt on each merged pull request

[teamleaderleo]

Why

main behaves like nightly: pull requests merge fast and conflict fast, and fixing breakage quickly matters more than blocking merges. But nobody records what CI had actually verified when a pull request landed. On 2026-09-25, 9 of 60 merged pull requests (15%) landed before any green ci-status on their head: #14461 merged while macOS compile admission was still running, and #14382 later broke SidebarAccessibilityTreeTests (also #14455 #14409 #14408 #14403 #14395 #14384 #14379).

What

• .github/workflows/merge-receipt.yml: pull_request_target on closed when merged, so fork pull requests get it too. It checks out only scripts/ci/merge_receipt.py at github.workflow_sha (never pull request code), reads the GitHub API, and runs after the merge, so it cannot block or slow one.
• scripts/ci/merge_receipt.py: reads the head commit's check runs and statuses (GraphQL statusCheckRollup), takes each check's state as of the merge time (finished before it, still running, or not started), groups the noise (guards jobs fold into one, app-host shards into one, bots and CLA hidden unless they failed), and posts one comment, edited in place on a re-run through a hidden marker. A judging check (macOS compile admission, app-host unit tests, ci-status, required checks) that was not green adds the new merged-unverified label; a re-run that finds it green removes it.
• scripts/ci/main_regression_attribution.py (ci: name the merged pull request behind each new main full-suite failure #14436): when suspects tie on score, pull requests labeled merged-unverified are listed first; the verified ones stay in the list, since a verified head can still break main through another merge. The label never creates a suspect on its own, and the existing caps are unchanged.
• Safety: only a github-actions comment carrying the marker is edited, and check names are escaped so a pull request's own job names cannot mention people or hide the receipt. Known limits (100-comment marker search, statuses show current state, merge queue would need gating) are noted in the script docstring.
• Tests: tests/test_ci_merge_receipt.py, fixture-driven from real snapshots of Wire AgentChatProseStreamWakeDriver.swift into the app target #14461 (unverified) and ci: weigh a package source change when an owned Mac picks its starting build #14433 (all green), no network. Registered on the linux-guard lane and in the guards ci group, with workflow_guard_groups.py owners for the script, workflow, fixtures, and the attribution script.

Sample receipt, dry run on #14461:

**Merge receipt** for `e9426f528e`, merged 2026-09-25 10:23:09 UTC
- Not verified at merge: ci-status (not reported), macOS compile admission (in progress)
- Verified: Web complexity, web-validation, Fast static checks, GhosttyKit release check, guards (17), linux-preflight, macOS admission gate, Testbox broker trust boundary
- Skipped by policy: browser, Claude wrapper regressions, remote-daemon, suite-coverage, swift-package-tests, web, web-build, web-database-tests, web-tests
- Full suite: runs on main after merge.

Labeled `merged-unverified`: if main breaks near this merge, look here first.

An all-green merge (#14433) gets one line: "every check was green at merge (14 verified; 15 skipped by policy). Full suite runs on main after merge."

How validated

• python3 tests/test_ci_merge_receipt.py (17 tests) and python3 tests/test_ci_main_regression_attribution.py (28 tests, 2 new) pass locally.
• python3 scripts/ci/validate_test_execution_registry.py passes.
• merge_receipt.py post --dry-run against Wire AgentChatProseStreamWakeDriver.swift into the app target #14461, Fix sidebar accessibility children cycle #14382, ci: weigh a package source change when an owned Mac picks its starting build #14433, ci: name the merged pull request behind each new main full-suite failure #14436 and ci(ios): charge in-flight iOS runs the simulators their title says they need #14437 printed the expected receipts.
• The merged-unverified label now exists in the repo.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 6f42d6c and f844576.

📒 Files selected for processing (10)

• .github/workflows/ci-guards.yml
• .github/workflows/merge-receipt.yml
• scripts/ci/main_regression_attribution.py
• scripts/ci/merge_receipt.py
• scripts/ci/workflow_guard_groups.py
• tests/fixtures/merge_receipt/pr14433.json
• tests/fixtures/merge_receipt/pr14461.json
• tests/test-execution.toml
• tests/test_ci_main_regression_attribution.py
• tests/test_ci_merge_receipt.py

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[github-actions]

Merge receipt for f8445764e0: every check was green at merge (8 verified; 11 skipped by policy). Full suite runs on main after merge.