feat: add visual companion to brainstorming skill

skills/brainstorming/visual-companion.md

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+# Visual Companion Reference
+
+## Starting the Server
+
+Run as a background job:
+
+```bash
+node ${PLUGIN_ROOT}/lib/brainstorm-server/index.js
+```
+
+Tell the user: "I've started a visual companion at http://localhost:3333 - open it in a browser."
+
+## Pushing Screens
+
+Write HTML to `/tmp/brainstorm/screen.html`. The server watches this file and auto-refreshes the browser.
+
+## Reading User Responses
+
+Check the background task output for JSON events:
+
+```json
+{"type":"user-event","type":"click","text":"Option A","choice":"optionA","timestamp":1234567890}
+{"type":"user-event","type":"submit","data":{"notes":"My feedback"},"timestamp":1234567891}
+```
+
+Event types:
+- **click**: User clicked button or `data-choice` element
+- **submit**: User submitted form (includes all form data)
+- **input**: User typed in field (debounced 500ms)
+
+## HTML Patterns
+
+### Choice Cards
+
+```html
+<div class="options">
+  <button data-choice="optionA">
+    <h3>Option A</h3>
+    <p>Description</p>
+  </button>
+  <button data-choice="optionB">
+    <h3>Option B</h3>
+    <p>Description</p>
+  </button>
+</div>
+```
+
+### Interactive Mockup
+
+```html
+<div class="mockup">
+  <header data-choice="header">App Header</header>
+  <nav data-choice="nav">Navigation</nav>
+  <main data-choice="main">Content</main>
+</div>
+```
+
+### Form with Notes
+
+```html
+<form>
+  <label>Priority: <input type="range" name="priority" min="1" max="5"></label>
+  <textarea name="notes" placeholder="Additional thoughts..."></textarea>
+  <button type="submit">Submit</button>
+</form>
+```
+
+### Explicit JavaScript
+
+```html
+<button onclick="brainstorm.choice('custom', {extra: 'data'})">Custom</button>
+```