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>

package.json

@@ -1,14 +1,43 @@
 {
-  "name": "ponytail",
+  "name": "opencode-ponytail",
   "version": "4.8.1",
   "description": "Lazy senior dev mode for AI agents. The best code is the code you never wrote.",
-  "keywords": ["pi-package", "pi", "skills", "ponytail"],
+  "keywords": ["opencode-plugin", "opencode", "ponytail", "pi-package", "pi", "skills"],
   "license": "MIT",
+  "author": {
+    "name": "Dietrich Gebert",
+    "url": "https://github.com/DietrichGebert"
+  },
+  "homepage": "https://github.com/DietrichGebert/ponytail",
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/DietrichGebert/ponytail.git"
+  },
+  "bugs": {
+    "url": "https://github.com/DietrichGebert/ponytail/issues"
+  },
+  "main": "./.opencode/plugins/ponytail.mjs",
+  "exports": {
+    ".": "./.opencode/plugins/ponytail.mjs",
+    "./plugin": "./.opencode/plugins/ponytail.mjs"
+  },
+  "files": [
+    "AGENTS.md",
+    "hooks/",
+    "skills/",
+    ".opencode/",
+    "pi-extension/",
+    "assets/",
+    "LICENSE"
+  ],
   "scripts": {
     "test": "node --test tests/*.test.js && npm test --prefix pi-extension"
   },
   "pi": {
     "extensions": ["./pi-extension/index.js"],
     "skills": ["./skills"]
+  },
+  "publishConfig": {
+    "access": "public"
   }
 }