refactor: improve programmatic API code quality

[tmchow]

Summary

Applied all 4 code quality improvements from #1076 to src/index.ts.

Changes

1. Include baseDir in error message - the checkRulesyncDirExists error now shows which directory failed
2. Extract BaseOptions type - shared properties (configPath, verbose, silent, global) extracted into a common type that GenerateOptions and ImportOptions extend
3. Explicit return types - added Promise<GenerateResult> and Promise<ImportResult> return type annotations to the public API functions
4. Explicit features destructuring - features is now explicitly destructured in importFromTool and passed to ConfigResolver.resolve, making the data flow clearer

Testing

All changes are type-level or error message improvements. No behavioral changes to the API functions.

Fixes #1076

This contribution was developed with AI assistance (Claude Code).

[dyoshikawa]

@tmchow thank you. please check ci failure.

[dyoshikawa]

@tmchow Thank you!

[tmchow]

Thanks for taking a look and merging!