docs(legibility): fix two blockers in CONTRIBUTING.md

- Clarify docs/ARCHITECTURE.md link with interim pointer to
  docs/architecture/c4-diagrams.md until DOC-2 PR merges
- Remove ./mvnw checkstyle:check — no checkstyle plugin in pom.xml;
  replace with ./mvnw test and ./mvnw clean package -DskipTests

Refs #398
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CONTRIBUTING.md

@@ -2,7 +2,7 @@
 
 For the full collaboration rules (issue workflow, PR process, Red/Green TDD, commit conventions) see [COLLABORATING.md](./COLLABORATING.md).
 For coding style see [CODESTYLE.md](./CODESTYLE.md).
-For the system architecture see [docs/ARCHITECTURE.md](./docs/ARCHITECTURE.md).
+For the system architecture see [docs/ARCHITECTURE.md](./docs/ARCHITECTURE.md) (introduced in DOC-2; until that PR merges, see [docs/architecture/c4-diagrams.md](./docs/architecture/c4-diagrams.md)).
 For domain terminology see [docs/GLOSSARY.md](./docs/GLOSSARY.md).
 
 ---
@@ -299,6 +299,7 @@ cd frontend && npm run lint     # Prettier + ESLint check
 cd frontend && npm run format   # Auto-fix formatting
 cd frontend && npm run check    # svelte-check (type errors)
 
-# Backend — checkstyle is enforced via Maven
-cd backend && ./mvnw checkstyle:check
+# Backend — no standalone lint tool; compilation and test runs catch style issues
+cd backend && ./mvnw test       # compile + test
+cd backend && ./mvnw clean package -DskipTests  # compile-only check
 ```