chore: remove core Java plugin

[GRAMMAC1]

Summary

• Remove the built-in Java diagnosis plugin from @error2fix/core.
• Keep the default core registry focused on TypeScript/frontend-oriented diagnosis plus generic fallback.
• Remove non-frontend workspace candidate files from core input collection.
• Update the benchmark report after the diagnosis behavior change.

Validation

• pnpm verify
• pre-commit Biome check during commit

Note: the first sandboxed pnpm verify run reached tests successfully but failed at the harness step because tsx could not create its IPC pipe under the sandbox. The non-sandbox rerun passed.