ci: don't cancel in-progress main push runs (fix startup_failure on rapid merges)

[JSONbored]

Problem

Merging two PRs within seconds left main's CI showing startup_failure on both push runs. Root cause: ci.yml uses

concurrency:
  group: ci-${{ github.ref }}
  cancel-in-progress: true

For push to main, github.ref is refs/heads/main, so back-to-back merges land in the same group and cancel-in-progress races them during startup → both end as startup_failure (GitHub can't cleanly cancel a run that's still starting). The workflows are valid — the PRs' pull_request runs passed identically.

Fix

Scope cancellation to PR events only:

cancel-in-progress: ${{ github.event_name == 'pull_request' }}

PR pushes still cancel superseded runs (fast feedback), but every merge to main now gets a complete CI run. Merging this also produces the clean green push run main is currently missing.

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🛠️ Maintainer review — Held for maintainer review

Reviewed 1 changed file(s) — two independent AI reviewers, synthesized. ✅ Safe to merge — both reviewers found no blocking issues.

ℹ️ Correct and all CI green — held for maintainer review.

Merge readiness: ✅ Mergeable · all CI checks green/neutral/skipped.

Summary

The change correctly scopes the concurrency cancel‑in‑progress flag to PR runs only, preventing cancellation of back‑to‑back main pushes and fixing the reported startup_failure issue. No security concerns, code correctness, or test regressions are introduced. The workflow syntax is valid and the added comments improve maintainability.

💡 Nits — non-blocking, optional

• Minor comment formatting could be tightened.
• Consider adding quotes around the expression (e.g., "${{ github.event_name == 'pull_request' }}") for clarity, though not required.
• Document the behavior in the project’s CI guidelines to inform contributors.

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Note

Gittensory Gate skipped

PR closed before full evaluation. No late first comment was created.

Signal	Result	Evidence	Action
Gate result	⚠️ Skipped	#1006 is no longer open.	No action.

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[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

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Held for maintainer (guardrail_area) — correct + CI green