CI: coverage gate 80% blocking on app/modules

[wab]

Parent

#121

What to build

Enable the 80% coverage threshold as a blocking PR gate on app/modules/**. HITL: requires a policy decision on what to exempt (legacy / generated / barrel files) and whether to also gate app/routes/** and app/components/**.

Acceptance criteria

• CI runs pnpm test --coverage on PR
• Threshold 80% (branches / functions / lines / statements) enforced on app/modules/**
• Exemptions documented in vitest.config.ts with rationale per file
• PR fails red when threshold drops
• Decision recorded on whether to extend gate to app/routes/** / app/components/**

Blocked by

• Test: RR v7 loader harness + homepage fallback loader coverage #131, Test: cache.ts coverage (TTL / SWR / bypass / Edge headers) #132, Test: github.ts MSW integration coverage #133, Test: processor.ts + Markdoc config.ts coverage #140, Test: i18n routing (lang param / fallback / redirects) #141 (most module + loader coverage needs to be in place first)