feat: publish ponytail as an installable npm package for OpenCode and Pi (#197)

* feat: register slash commands from .opencode/command/*.md

Add parseCommandFile() to read frontmatter-described markdown files
and wire them into opencode's config.command during init.

Extend the config hook to scan .opencode/command/ and register each
.md file as a named slash command.

Update the plugin doc comment to reflect the npm install path
(opencode-ponytail) vs the old relative path.

* chore: rename package to opencode-ponytail

Align package name with npm convention for opencode plugins.
Update keywords to include opencode-plugin and opencode tags.
Fix description back to original correct wording (grammatical
regression introduced during editing).

* chore: add npm metadata and publish workflow

Add author, homepage, repository, bugs, main, exports, files, and
publishConfig fields to package.json for npm publishing.

Add .github/workflows/publish.yml to auto-publish to npm on version
tags (v*) with provenance.

* docs: add npm plugin install for opencode-ponytail

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Co-authored-by: Emeriko <dietrich.gebert@gmail.com>