feat: add Ruby diagnostics support via rubocop

[siddhanttiwari19]

Summary

Adds .rb support to the post-edit diagnostics feedback loop in
src/tools/diagnostics.ts. Follows the existing Python branch pattern
exactly — checks onPath("rubocop") before running, returns null
silently when rubocop is absent, never throws, never blocks.

Linked issue

Refs #1

How I verified it

• bun run typecheck — no type errors
• bun test — 47/47 passing across 8 files, 0 failures
• bun run build — clean build (0.34 MB, 70 modules)

[github-actions]

🤖 KlaatAI Review Bot (powered by Klaatu, advisory only — a maintainer makes the real call)

Issue match
Fully addresses issue #1. It adds the exact .rb branch in commandFor() using the requested rubocop arguments and correctly follows the existing Python pattern.

Test coverage
src/tools/diagnostics.test.ts adds 5 tests covering the negative paths (rubocop absent, diagnostics disabled, empty commands config, non-.rb unaffected). It lacks coverage for the happy path where rubocop is actually on PATH. A missing test should mock onPath("rubocop") to return true and assert that commandFor returns the exact expected command array.

Correctness concerns
None. The logic in src/tools/diagnostics.ts:72-75 safely handles the missing binary via onPath(), uses the correct absPath, and will not throw or block.

Verdict
Ready to merge as-is, but flag to a human reviewer whether the lack of a mocked happy-path test for rubocop being present is acceptable for this PR.

_{This is an automated review to help triage faster, not a gate. Nothing here blocks merging.}