Guard: block new '*-coverage.test.ts' bolt-on files

[JSONbored]

Part of the '-coverage' bolt-on consolidation epic (#8574 — non-closing reference). Do this LAST, after the existing offenders are consolidated, so the check lands green.

The bolt-on pattern is a direct side effect of the 99% Codecov patch gate: a contributor's cheapest path to green is a new file of generic "exercises X branches" tests rather than extending the module's existing suite. Once the existing family is gone, prevent recurrence:

Task

• Add a drift-style CI check (alongside the existing docs:drift-check / manifest:drift-check family in test:ci) that fails when a file matching test/**/*-coverage.test.ts or test/**/*-branch-coverage.test.ts exists, with an allowlist for legitimate names (rees-coverage-script.test.ts — it tests scripts/rees-coverage.js, actual coverage tooling).
• Update .claude/skills/contributing-to-loopover/ (SKILL.md Phase 3 and/or reference.md test-patterns section) to say explicitly: coverage gaps are closed by extending the module's existing test file, never by adding a -coverage-named bolt-on file.
• Wire the check into npm run test:ci so it's part of the local gate too.

[linear-code]

JSO-5945