fix: allow searchPath to target files and directories

[bmdavis419]

Summary

• allow file targets in git resource searchPath validation/runtime checks
• update user-facing validation hints
• add test coverage for file-based targets

Issues

• Closes Add ability to include specific files in searchPaths #189

Greptile Summary

Extended git resource searchPath validation to support both files and directories, fixing issue #189.

Key Changes:

• Replaced directoryExists with new pathExists helper to validate both file and directory targets
• Updated error hints in getSearchPathHint to mention files as valid targets (e.g., 'src/docs' or 'README.md')
• Added integration test coverage for file-based searchPath using README.md as test case
• Improved git error messages for better user guidance

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The change is backward compatible (directories still work), adds test coverage for the new file support, and aligns with git sparse-checkout's native support for both files and directories
• No files require special attention

Important Files Changed

Filename	Overview
apps/server/src/resources/impls/git.ts	Added pathExists helper and updated validation to support both files and directories in searchPath
apps/server/src/resources/impls/git.test.ts	Added integration test to verify file-based searchPath support with README.md

_{Last reviewed commit: c1f0a52}