fix: choose containing tsconfig for monorepo files

[Egoistian]

Summary

• extract tsconfig selection into a small pure helper
• choose the closest tsconfig whose directory contains the target file
• avoid selecting a sibling package tsconfig when a monorepo package has no local tsconfig
• add coverage for nearest-match, sibling fallback, node_modules, and Windows-style paths

Why

The previous prefix-based lookup sorted tsconfig files by path length, then returned the first config whose path started with one of the target file's ancestor prefixes. In a monorepo, that can select a sibling package's tsconfig when the target package does not have its own config.

Example:

• target file: /repo/packages/app-b/src/index.ts
• configs: /repo/tsconfig.json, /repo/packages/app-a/tsconfig.json

The sibling /repo/packages/app-a/tsconfig.json should not apply to app-b; the workspace root config is the safer fallback.

Verification

• pnpm typecheck
• pnpm lint
• pnpm exec vitest run
• pnpm build