docs: consolidate documentation into wiki

- Simplify README to essentials with links to wiki
- Move detailed content to wiki pages:
  - Architecture (shim system, version resolution, plugin system)
  - Migration (from nvm, pyenv, fnm)
  - Development (full guide with commit conventions, CI/CD)
- Remove docs/COMMIT_CONVENTION.md (merged into wiki Development)
- README reduced from ~700 lines to ~115 lines

Author: CalvinAllen