feat(detect): add Java Gradle defaults

[mbelinky]

Summary

• Detect Java language from common source roots.
• Infer root Gradle project defaults for build and test commands, preferring ./gradlew when present.
• Update docs to mention Java/Kotlin Gradle coverage.

Why

Gradle Java projects already map source and test groups, but project detection did not report Java or default Gradle validation commands. This makes mapped Gradle projects more actionable for review runs.

Validation

• pnpm typecheck
• pnpm test -- src/mapper.test.ts
• pnpm lint
• pnpm build
• Local dry-run against a Java/Gradle repository confirmed Java detection, wrapper build/test commands, and stable mapped review features

Red-team notes

• Java detection is limited to common source roots so documentation-only .java snippets do not mark a project as Java.
• Gradle defaults are only inferred for root Gradle projects, so nested Gradle apps inside another root do not get incorrect top-level commands.
• This does not add framework-specific Java semantic role mapping.