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

settings.json

@@ -1,12 +1,9 @@
 {
+  "attribution": {
+    "commit": "",
+    "pr": ""
+  },
   "permissions": {
-    "ask": [
-      "Bash(make dev:*)",
-      "Bash(git merge:*)",
-      "Bash(git branch -d:*)",
-      "Bash(git branch -D:*)",
-      "Bash(git branch --delete:*)"
-    ],
     "allow": [
       "Bash(git:*)",
       "Bash(npm:*)",
@@ -33,6 +30,13 @@
       "Skill(superpowers:*)",
       "Skill(frontend-design:*)",
       "WebSearch"
+    ],
+    "ask": [
+      "Bash(make dev:*)",
+      "Bash(git merge:*)",
+      "Bash(git branch -d:*)",
+      "Bash(git branch -D:*)",
+      "Bash(git branch --delete:*)"
     ]
   },
   "statusLine": {
@@ -41,11 +45,8 @@
   },
   "enabledPlugins": {
     "superpowers@superpowers-marketplace": true,
-    "frontend-design@claude-code-plugins": true
+    "frontend-design@claude-code-plugins": true,
+    "example-skills@anthropic-agent-skills": true
   },
-  "alwaysThinkingEnabled": true,
-  "attribution": {
-    "commit": "",
-    "pr": ""
-  }
+  "alwaysThinkingEnabled": true
 }