[BUG] EEXIST when overwriting an existing skill during installation

Bug Description When attempting to install a skill that is already present in the project, selecting "Yes" to overwrite causes the CLI to crash with an EEXIST error. It seems the internal copy operation cpSync fails because the destination directory already exists and is not being handled correctly during the overwrite process.

Steps to Reproduce

1. mkdir test-repo && cd test-repo
2. npx openskills install nextlevelbuilder/ui-ux-pro-max-skill
3. Run the exact same install command again: npx openskills install nextlevelbuilder/ui-ux-pro-max-skill
4. When prompted "Skill 'ui-ux-pro-max' already exists. Overwrite?", select y.

Expected Behavior The existing skill directory should be overwritten or updated with the fresh copy from the repository without crashing.

Actual Behavior The process crashes immediately after selecting "Yes" with a Node.js EEXIST error.

Environment

• openskills version: 1.5.0 (via npx openskills --version)
• Operating System: macOS 26.3
• Node.js version: v24.13.0
• AI agent: Claude Code (Targeting .claude/skills)

Installation Context

• Global install (npm i -g openskills)
• Local install via npm link
• Installed skills: (list from openskills list)
• npx execution (npx openskills ...)

Additional Context full error log & stack trace:

✔ Skill 'ui-ux-pro-max' already exists. Overwrite? Yes
node:internal/fs/cp/cp-sync:145
    return fsBinding.cpSyncCopyDir(src, dest,
                     ^
Error: EEXIST, File exists '/Users/yuki/workspace/test-repo/.claude/skills/ui-ux-pro-max'
    at copyDir (node:internal/fs/cp/cp-sync:145:22)
    at onDir (node:internal/fs/cp/cp-sync:137:10)
    at getStats (node:internal/fs/cp/cp-sync:68:12)
    at cpSyncFn (node:internal/fs/cp/cp-sync:58:10)
    at cpSync (node:fs:3159:3)
    at installFromRepo (file:///Users/yuki/.npm/_npx/3527d6942a6d9730/node_modules/openskills/dist/cli.js:472:5)
    at async Command.installSkill (file:///Users/yuki/.npm/_npx/3527d6942a6d9730/node_modules/openskills/dist/cli.js:279:7) {
  errno: 17,
  code: 'EEXIST',
  path: '/Users/yuki/workspace/test-repo/.claude/skills/ui-ux-pro-max',
  syscall: 'cp'
}