Add portable plugin configuration for cross-host sync

- Create plugins/config.json with marketplace sources and desired plugins
- Remove plugins/installed_plugins.json from tracking (host-specific paths)
- Add installed_plugins.json to .gitignore
- Update README with plugin installation instructions
- Enable example-skills plugin in settings.json

plugins/config.json

@@ -1,3 +1,29 @@
 {
-  "repositories": {}
-}
+  "repositories": {
+    "claude-plugins-official": {
+      "source": "github",
+      "repo": "anthropics/claude-plugins-official"
+    },
+    "anthropic-agent-skills": {
+      "source": "github",
+      "repo": "anthropics/skills"
+    }
+  },
+  "desired_plugins": [
+    {
+      "id": "superpowers@superpowers-marketplace",
+      "version": "4.0.0",
+      "gitCommitSha": "84283dfc058f61b8985e07d20eb6838a6e7388fe",
+      "note": "Marketplace source unknown - install manually if needed"
+    },
+    {
+      "id": "frontend-design@claude-code-plugins",
+      "version": "1.0.0",
+      "gitCommitSha": "5880baedc387d8bad189b4c778f69477e88feb7f",
+      "note": "Marketplace source unknown - install manually if needed"
+    },
+    {
+      "id": "example-skills@anthropic-agent-skills"
+    }
+  ]
+}

plugins/installed_plugins.json

@@ -1,27 +0,0 @@
-{
-  "version": 2,
-  "plugins": {
-    "superpowers@superpowers-marketplace": [
-      {
-        "scope": "user",
-        "installPath": "/home/bill/.claude/plugins/cache/superpowers-marketplace/superpowers/4.0.0",
-        "version": "4.0.0",
-        "installedAt": "2025-11-23T20:31:21.915Z",
-        "lastUpdated": "2025-12-21T16:54:24.259Z",
-        "gitCommitSha": "84283dfc058f61b8985e07d20eb6838a6e7388fe",
-        "isLocal": false
-      }
-    ],
-    "frontend-design@claude-code-plugins": [
-      {
-        "scope": "user",
-        "installPath": "/home/bill/.claude/plugins/cache/claude-code-plugins/frontend-design/1.0.0",
-        "version": "1.0.0",
-        "installedAt": "2025-11-24T21:24:25.872Z",
-        "lastUpdated": "2025-11-24T21:24:25.872Z",
-        "gitCommitSha": "5880baedc387d8bad189b4c778f69477e88feb7f",
-        "isLocal": true
-      }
-    ]
-  }
-}