ci(pr-gate): sync slash-menu and runner fixes from EOS #21

[autogame-17]

Touches hot-path: no
Phase: fix
Risk vector: forward

Summary

Syncs pr metadata policy from upstream EvoMap/engineering-operating-system (PR #21), which fixes two defects that block PRs for reasons unrelated to PR quality. Both have to land here before the check can be made required.

1. Slash-separated template menus are now recognised as menus. The gate derives its contract from this repo's own PR template, including which answers a Label: a | b | c line offers. The old parser only understood the pipe form, so a template writing the same menu with slashes offered nothing and a placeholder-looking word the template explicitly listed was rejected. EvoMap/evox writes Builds on: <#PR / #issue / docs/… path / none> and documents none for genuinely fresh work — the gate rejected that exact answer. Both separators now parse; the separator must be a spaced slash so a path like docs/… path is not shredded into bogus choices, and an et-cetera entry (..., …, etc) is not treated as a selectable answer.

2. The gate no longer inherits the org's general CI runner label. runs-on read vars.CI_LABEL_LINUX_X64, which resolves to [self-hosted, Linux, X64] on this org. That queued a three-second check behind heavy builds on a shared autoscaled pool, and jobs came back cancelled with not acquired by Runner of type self-hosted even after multiple attempts. For a required check, a job that never reaches a conclusion blocks the PR exactly as hard as a failing one, and re-running does not converge reliably. The gate now defaults to ubuntu-latest; opting into self-hosted capacity is a dedicated PR_GATE_RUNNER variable, so no repo inherits a runner dependency merely by belonging to an org with a fleet.

Scope and cuts

PR authors in this repo now get a metadata verdict that reflects their PR body: the answer this repo's own template prescribes is accepted, and the check reaches a conclusion instead of dying unacquired.

Nothing cut. This is the complete upstream sync of both fixes.

Not in scope: adding pr metadata policy to this repo's branch protection. That is a separate, owner-visible step and is deliberately gated on this landing first.

Module boundary

CI configuration only, and only the files the pr-gate module owns:

• .github/workflows/pr-metadata-policy.yml
• scripts/validate_pr_metadata.py

No product code, no tests, no build config, no PR template change. The contract this gate enforces is unchanged — it is still derived from this repo's own .github/PULL_REQUEST_TEMPLATE.md on the default branch. No existing PR body needs editing because of this change.

Alternative considered

Wait for the next routine EOS sync instead of a targeted PR. Rejected: the gate is being promoted to a required check, and shipping that promotion on top of the known-bad validator is what would strand PRs whose Builds on: is the template-documented none.

Pinning the job to a narrower self-hosted label instead of moving to GitHub-hosted was also considered upstream and rejected — it still depends on the pool that demonstrably fails to acquire work. Using continue-on-error to paper over the starvation was rejected as strictly worse: it converts a starved gate into a silently-passing one.

Validation

• Upstream EvoMap/engineering-operating-system#21: python3 -m unittest discover -s tests — 103 tests, 1 skipped, pass; python3 scripts/validate_scaffold.py — scaffold ok.
• Differential proof across the live fleet: the pre-fix and post-fix validators were run against all 189 open PRs in the 19 adopting repos, each against its own default-branch template. Exactly one verdict change (EvoMap/evox#3809 FAIL → PASS, the bug) and zero PASS → FAIL regressions — so this sync cannot newly fail any currently-passing PR in this repo.
• Files here are byte-identical to upstream canonical (pr-gate/validate_pr_metadata.py and pr-gate/pr-metadata-policy.yml), which upstream's validate_scaffold.py enforces.
• This PR's own pr metadata policy check exercises the synced workflow against this repo's real template.

Rollback / recovery

Revert this PR to return to the previous workflow and validator. Before reverting, remove pr metadata policy from this repo's branch-protection required checks if it has been added — reverting restores both the Builds on: none rejection and the self-hosted starvation, either of which permanently blocks PRs once the check is required.

Builds on: none

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Note

Medium Risk
Changes how required PR metadata is validated and where the gate runs; mis-parsing menus or runner misconfiguration could block or silently weaken enforcement, though upstream differential testing reported one targeted fix and no PASS→FAIL regressions.

Overview
Syncs the pr metadata policy gate from upstream EOS so two blocking defects are fixed before the check can be made required.

The workflow runs-on no longer uses the org-wide CI_LABEL_LINUX_X64 self-hosted label. It defaults to ubuntu-latest, with optional override via PR_GATE_RUNNER, so a short stdlib-Python job is less likely to stall or cancel with runner acquisition failures when the check must always reach a conclusion.

validate_pr_metadata.py extends _offered_choices so declaration menus written as a / b / c (not only pipe-separated lists) count as offered answers—e.g. Builds on: … / none—while stripping angle brackets, requiring a spaced slash separator, and ignoring continuation tokens like ... via CONTINUATION_RE so they are not valid selectable answers.

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