Add Claude Code provider plugin to OpenCode

[CollieIsCute]

Summary

• add @khalilgharbaoui/opencode-claude-code-plugin@0.5.1 to OpenCode plugins
• keep model selection manual in OpenCode instead of forcing model/small_model defaults

Security

• no Claude token, API key, URL, or auth material is committed
• this relies on the local Claude Code CLI already being initialized by the user
• no claude commands were run during verification

Verification

• npm metadata/README reviewed for plugin configuration
• opencode config schema checked for model/small_model behavior
• chezmoi execute-template < home/dot_config/opencode/opencode.json.tmpl | jq . > /dev/null
• checked final diff for Redmine URL and secret-looking keys