Use directory copy instead of symlinks for skills on Windows

[svarlamov]

Use directory copy instead of symlinks for skills on Windows

Summary

Resolves #533. On Windows, creating symlinks requires Administrator privileges for legacy reasons. This PR updates the skills installer to copy skill directories on Windows instead of creating symlinks, removing the admin requirement. Unix behavior (symlinks) is unchanged.

Changes in src/mdm/skills_installer.rs:

• create_skills_symlink → link_skill_dir: uses #[cfg(unix)] symlink / #[cfg(windows)] recursive copy
• New copy_dir_recursive (Windows-only): recursively copies a skill directory and its contents
• remove_skills_symlink → remove_skill_link: now removes both symlinks (existing Unix installs) and copied directories (Windows), so uninstall works correctly on both platforms

Updates since last revision

Added unit tests covering the skills installation and linking logic (11 new tests total):

• link_skill_dir: content accessibility, replacing existing directories/files, parent directory creation, symlink verification on Unix
• remove_skill_link: directory removal, symlink removal, no-op on nonexistent paths
• test_install_and_uninstall_skills_lifecycle: full install → re-install → uninstall → noop-uninstall lifecycle

Note: These tests exercise the Unix (symlink) code path. The Windows copy_dir_recursive path is verified by compilation only — it cannot be exercised in the Linux/macOS test runners.

Review & Testing Checklist for Human

• Verify on a real Windows machine: the #[cfg(windows)] code paths (copy_dir_recursive, the copy branch in link_skill_dir) cannot be tested on Linux CI. Run git-ai install-hooks on Windows without admin privileges and confirm skills appear correctly in ~/.agents/skills/ and ~/.claude/skills/
• Verify uninstall on Windows: run uninstall and confirm copied skill directories are cleaned up
• Review remove_skill_link safety: the new logic calls remove_dir_all on non-symlink directories at the skill link paths — confirm this is acceptable given the paths are always ~/.agents/skills/{skill-name} or ~/.claude/skills/{skill-name}

Notes

• Requested by: @svarlamov
• Link to Devin run

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.

[svarlamov]

Are there unit tests covering the skill installation? If not, can you add some to verify functionality and prevent regresions?

[devin-ai-integration]

The existing tests only check that embedded skills are loaded and the skills dir path is correct. I'll add tests covering the actual install/uninstall logic including the linking and cleanup behavior.