The repository is private, so anonymous HTTPS clone used by
`claude plugin marketplace add https://github.com/...`

 fails with
"Repository not found". New installers need collaborator access plus
SSH key setup before the plugin commands will work.

Adds a "사전 준비: 저장소 접근 권한" section covering:
- Requesting collaborator invite from the repo owner
- Generating an ed25519 SSH key and registering the public half on GitHub
- Verifying connectivity with `ssh -T git@github.com`

Switches the marketplace / clone snippets to SSH URLs
(`git@github.com:hybk96/oh-my-elastiCore.git`) which work for private
repos, and notes the HTTPS alternative for if/when the repo becomes
public. Adds a troubleshooting hint under 업데이트 for the common
"Permission denied (publickey)" failure mode.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>