Add Claude Code plugin marketplace manifest

Closes #1 (Claude Code portion). The repo already ships as a single
plugin (root .claude-plugin/plugin.json); this adds the marketplace
catalog so it installs natively in Claude Code, leaving the existing
npx skills flow untouched.

- .claude-plugin/marketplace.json: lists the plugin via a github source
  pointing at this repo; version omitted so installs track the latest
  commit
- README: "As a Claude Code plugin" install steps

Verified with `claude plugin marketplace add` (accepted and listed).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -0,0 +1,22 @@
+{
+  "name": "effective-html",
+  "owner": {
+    "name": "plannotator"
+  },
+  "description": "Agent skills for elegant, self-contained HTML plans, diagrams, and artifacts.",
+  "plugins": [
+    {
+      "name": "plannotator-effective-html",
+      "source": {
+        "source": "github",
+        "repo": "plannotator/effective-html"
+      },
+      "description": "HTML skills for pragmatic visual artifacts — html, html-diagram, and html-plan.",
+      "homepage": "https://github.com/plannotator/effective-html",
+      "repository": "https://github.com/plannotator/effective-html",
+      "license": "MIT",
+      "keywords": ["html", "diagram", "plan", "svg", "artifacts", "skills"],
+      "category": "productivity"
+    }
+  ]
+}

README.md

@@ -44,6 +44,15 @@ npx skills add plannotator/effective-html --skill html-diagram
 npx skills add plannotator/effective-html --skill html-plan
 ```
 
+### As a Claude Code plugin
+
+This repo is also a [plugin marketplace](https://code.claude.com/docs/en/plugin-marketplaces). Add it and install:
+
+```
+/plugin marketplace add plannotator/effective-html
+/plugin install plannotator-effective-html@effective-html
+```
+
 ## Skills
 
 - `html` - Create an HTML file for whatever the user is describing, matching the effective HTML references.