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

---------

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

.github/workflows/publish.yml

@@ -0,0 +1,23 @@
+name: publish
+
+on:
+  push:
+    tags: ['v*']
+  workflow_dispatch:
+
+permissions:
+  id-token: write
+  contents: read
+
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '22'
+          registry-url: 'https://registry.npmjs.org'
+      - run: npm publish --provenance --access public
+        env:
+          NODE_AUTH_TOKEN: ${{ secrets.NPM_TOKEN }}