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

[dangtrungan]

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.

[dangtrungan]

@DietrichGebert Alongside with the new node package, I've also set up OIDC for the release workflow

[accodev]

This would be very helpful, thanks!

[accodev]

@dangtrungan hey man why was the PR closed?

[dangtrungan]

I f*cked up. I will create a PR again.

[vasilvestre]

Done here #197