Fix Antigravity support (#32)

* Fix Antigravity skills path

Update the default install path for Antigravity from:
  ~/.gemini/antigravity/global_skills
to:
  ~/.gemini/antigravity/skills

This aligns with the official Antigravity documentation:
https://antigravity.google/docs/skills

* Add --copy flag for installing skills without symlinks

Some AI tools don't support symbolic links for skills. This adds a --copy
flag that creates full copies instead of symlinks.

Changes:
- Add --copy/-c flag to install command
- Add copyItem to FileSystem protocol
- Implement copyItem in InMemoryFileSystem for tests
- Add test for copy installation
- Document when to use --copy in README

Usage:
  pfw install --tool <tool> --copy

Note: Using --copy duplicates skills for each tool and requires
re-running install after updates.

* Remove dead code in InMemoryFileSystem.copyItem

The copyItem method had code handling files and symlinks as source,
but in practice we always copy directories from ~/.pfw/skills/.
Removed unreachable code paths for clarity.

* Remove symlink copying in copyItem

The source directory (~/.pfw/skills/) never contains symlinks,
so there's no need to copy them.

* Copy only for cursor.

* Update readme.

* Remove Cursor, add Antigravity

* fix test

---------

Co-authored-by: Brandon Williams <mbrandonw@hey.com>
Co-authored-by: Stephen Celis <stephen@stephencelis.com>

Author: 3 people

Package.resolved

@@ -12,6 +12,7 @@ protocol FileSystem: Sendable {
   func data(at url: URL) throws -> Data
   func createSymbolicLink(at url: URL, withDestinationURL destURL: URL) throws
   func moveItem(at srcURL: URL, to dstURL: URL) throws
+  func copyItem(at srcURL: URL, to dstURL: URL) throws
   func contentsOfDirectory(at url: URL) throws -> [URL]
   func unzipItem(at sourceURL: URL, to destinationURL: URL) throws
 }

Sources/pfw/Dependencies/FileSystem.swift

@@ -225,8 +225,13 @@ struct Install: AsyncParsableCommand {
       let centralSkillDirectories =
         (try? fileSystem.contentsOfDirectory(at: centralSkillsURL)) ?? []
       for directory in centralSkillDirectories {
+        try fileSystem.write(Data("*\n".utf8), to: directory.appendingPathComponent(".gitignore"))
         let toolDestination = skillsURL.appendingPathComponent("pfw-\(directory.lastPathComponent)")
-        try fileSystem.createSymbolicLink(at: toolDestination, withDestinationURL: directory)
+        if target.tool == .antigravity {
+          try fileSystem.copyItem(at: directory, to: toolDestination)
+        } else {
+          try fileSystem.createSymbolicLink(at: toolDestination, withDestinationURL: directory)
+        }
       }
       if let tool = target.tool {
         print("  • \(tool.rawValue): \(skillsURL.path)")