feat: add Windows support for skills symlinks and secret key permissions

- Add Windows symlink support in skills/mod.rs with fallback chain:
  1. symlink_dir (requires admin/developer mode)
  2. mklink /J junction (works without admin)
  3. copy_dir_recursive fallback
- Add Windows file permissions in security/secrets.rs using icacls
- Add copy_dir_recursive helper function for non-Unix platforms

Fixes #28

src/security/secrets.rs

@@ -181,13 +181,22 @@ impl SecretStore {
             fs::write(&self.key_path, hex_encode(&key))
                 .context("Failed to write secret key file")?;
 
-            // Set restrictive permissions (Unix only)
+            // Set restrictive permissions
             #[cfg(unix)]
             {
                 use std::os::unix::fs::PermissionsExt;
                 fs::set_permissions(&self.key_path, fs::Permissions::from_mode(0o600))
                     .context("Failed to set key file permissions")?;
             }
+            #[cfg(windows)]
+            {
+                // On Windows, use icacls to restrict permissions to current user only
+                let _ = std::process::Command::new("icacls")
+                    .arg(&self.key_path)
+                    .args(["/inheritance:r", "/grant:r"])
+                    .arg(format!("{}:F", std::env::var("USERNAME").unwrap_or_default()))
+                    .output();
+            }
 
             Ok(key)
         }