ref: Set no-unused-vars eslint rule to error and clean up violations

[AbhiPrasad]

resolves #1406

AI Summary

why this change is important:

The @typescript-eslint/no-unused-vars rule catches dead code — unused imports, variables, and type definitions that add noise, increase bundle size (for non-type imports), and make the codebase harder to maintain. Promoting it from "warn" to "error" means:

1. It blocks CI — unused variables can no longer slip in unnoticed. Previously warnings were easy to ignore.
2. Cleaner imports — many files were importing Zod runtime schemas (e.g., CodeBundleSchema, RepoInfoSchema) that were never used, only their TypeScript types were needed. These unused runtime imports could pull in unnecessary code.
3. Removes dead code — things like serializeModel, SdkMcpToolDefinition, and HookEvent were defined but never referenced anywhere, creating confusion about whether they're part of the API surface.
4. Prevents regressions — as an error, any future unused variable is caught at lint time rather than accumulating as tech debt.