perf(ci): gate coverage collection to merge_group only — follow-up to #363

Parent: #363. Spun from #363's DoD by the orchestrator; **needs maintainer `approved` (ADR-003)**.

## Goal
Skip `collectCoverage` on the high-frequency `pull_request` event; keep `coverageThreshold`/`fail_under` enforced on `merge_group` so nothing merges under-covered.

## Scope
- Conditional coverage on event name (PR = off, merge_group = on).
- Verify the required `build` context still reports on PRs without coverage.

Refs #363

Start by locating the CI workflow entries for collectCoverage, coverageThreshold, and fail_under, then compare the pull_request and merge_group event paths. Verify that coverage runs only for merge_group and that the required build context still reports on pull_request; maintainer approval under ADR-003 is also required.

Written by the indexing model from the issue text.