fix: normalize path separators for cross-platform gitignore matching

[Copilot]

On Windows, filepath.Walk returns paths with backslashes (e.g., .agent\skills\test.txt), but gitignore pattern matching expects forward slashes. This caused .zeaburignore and .gitignore patterns to fail on Windows.

Changes:

• Normalize path separators to forward slashes with filepath.ToSlash() before matching against gitignore patterns in internal/util/pack.go
• Apply normalization before appending trailing slash for directory matching

// Before: Windows paths with backslashes failed to match
checkPath := path  // ".agent\skills\test.txt" on Windows

// After: Normalized to forward slashes for cross-platform matching
checkPath := filepath.ToSlash(path)  // ".agent/skills/test.txt"
if info.IsDir() {
    checkPath = checkPath + "/"
}

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[Copilot]

Pull request overview

This PR fixes a cross-platform compatibility issue where gitignore pattern matching failed on Windows due to path separator differences. The fix ensures that .zeaburignore and .gitignore patterns work correctly on Windows by normalizing backslashes to forward slashes before pattern matching.

Changes:

• Normalize path separators to forward slashes using filepath.ToSlash() before gitignore pattern matching in internal/util/pack.go

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