feat(opencode): publish ponytail as an installable npm package (opencode-ponytail)

## Summary Turns the OpenCode plugin into a standalone, npm-installable package (`opencode-ponytail`) rather than something that only works from a checkout of this repo, and switches slash command registration from a static config to one generated dynamically from the command files shipped inside the package. ## Changes - Renamed the package to `opencode-ponytail` and added the metadata npm expects for publishing (license, author, repository, homepage, bugs, keywords, version bump). - Updated `package.json` `files`/`exports` so command definitions, hooks, and skills are actually included when the package is published. - Updated the README with npm install instructions and clarified how to point `opencode.json` at a local checkout instead. ## Testing - Added `tests/opencode-package.test.js`: a smoke test that imports the package entry point the way npm consumers would and verifies it exports a function, injects the ruleset at the default mode, and persists `/ponytail off`. - Extended `tests/opencode-plugin.test.js` to verify the config hook registers all four slash commands (`ponytail`, `ponytail-review`, `ponytail-audit`, `ponytail-help`) with string `description`/`template` fields.