fix: respect gitignore in search results

[nagiexplorer88]

Fixes #35.

Summary

• Reads each workspace root .gitignore when search.useIgnoreFiles is enabled.
• Filters Storm Search candidate files against common .gitignore patterns before content search.
• Supports comments, negation, rooted rules, directory-only rules, and */?/** globs without adding a dependency.

Validation

• git diff --check
• npm run compile was blocked locally because this fresh clone has no installed dependencies and tsc is unavailable. I did not run npm install.

AI-assisted implementation, manually reviewed before submission.