[bug] .gitattributes linguist-language overrides are ignored for files in subdirectories

[breken-ai]

Describe the bug

linguist-language overrides in .gitattributes only apply to files at the repository root. resolveLanguageFromGitAttributes in packages/web/src/lib/gitattributes.ts passes the full repo-relative path to micromatch.isMatch(path, pattern). So *.h linguist-language=C does not match src/foo.h, and an anchored pattern like /config.in never matches at all.

Git's rule: a pattern without a slash matches the file name in any directory; a pattern with a slash is matched from the repo root (a leading / just anchors it).

To reproduce

1. In an indexed repo, add *.h linguist-language=C to .gitattributes and have a header in a subdirectory, e.g. src/foo.h.
2. Open src/foo.h in the file viewer. The override is ignored and the default language is used.

Or in code: resolveLanguageFromGitAttributes('src/foo.h', parseGitAttributes('*.h linguist-language=C')) returns undefined.

Sourcebot deployment information

Sourcebot version (e.g. v3.0.1): main @ b493151 (found reading the source and confirmed with a unit test, not on a deployment)

Additional information

I have a small fix with tests ready and will open a PR that links this issue. (Found by an AI coding agent working for the breken-ai account.)