Add Modern Web Guidance as the rung-3 lookup for web tasks (#82)

Scoped, optional reference to Modern Web Guidance so the agent can look
up native platform features on web work, filling the gap at rung 3 of
the ladder. Three additive changes, no compact-ruleset surgery:

- skills/ponytail/SKILL.md: "Web tasks: rung 3 lookup" section after the
  ladder. Runtime source only, not byte-compared, so no six-file sync.
- README.md: one "Pairs well with" line, matching the Caveman pattern.
- examples/web-platform-lookup.md: a <dialog closedby> vs Radix
  before/after in the date-picker.md style.

Lookup, not license: MWG suggests, the ladder filters. Absent CLI
changes nothing. No new INVARIANT phrase; rule-copy check stays green.

Co-authored-by: Claude <noreply@anthropic.com>

examples/web-platform-lookup.md

@@ -0,0 +1,64 @@
+# Web Platform Lookup
+
+**Task:** "Add a modal dialog that closes when you click the backdrop."
+
+Rung 3 of the ladder is "native platform feature covers it?" On web work the
+trap is that the agent forgets what the platform already does and reaches for a
+library. When ponytail has [Modern Web Guidance](https://github.com/GoogleChrome/modern-web-guidance)
+on hand, rung 3 gets a lookup: `modern-web search "modal dialog light dismiss"`.
+
+## Without Ponytail
+
+```bash
+npm install @radix-ui/react-dialog
+```
+
+```jsx
+import * as Dialog from "@radix-ui/react-dialog";
+
+export default function Modal({ open, onOpenChange, children }) {
+  return (
+    <Dialog.Root open={open} onOpenChange={onOpenChange}>
+      <Dialog.Portal>
+        <Dialog.Overlay className="overlay" />
+        <Dialog.Content className="content">
+          {children}
+          <Dialog.Close className="close">×</Dialog.Close>
+        </Dialog.Content>
+      </Dialog.Portal>
+    </Dialog.Root>
+  );
+}
+```
+
+A dependency, a portal, an overlay node, and controlled open state, to put a
+box on top with a backdrop.
+
+## With Ponytail
+
+`modern-web search "modal dialog light dismiss"` →
+`modern-web retrieve light-dismiss-a-dialog`. The platform has it:
+
+```html
+<!-- ponytail: <dialog> + closedby, browser does the backdrop, focus trap, and Esc -->
+<dialog closedby="any">
+  <p>...</p>
+</dialog>
+```
+
+```js
+document.querySelector("dialog").showModal();
+```
+
+**1 dependency + portal/overlay machinery → 0 dependencies + a `<dialog>`.**
+The `::backdrop` is free, focus is trapped and restored for you, `Esc` closes
+it, and `closedby="any"` adds click-outside dismissal. The browser team did the
+work.
+
+## The point
+
+MWG suggests the cutting edge, ponytail keeps only the rung that holds. The
+lookup found `light-dismiss-a-dialog`; the ladder took it because it deletes a
+dependency. The same search would have offered scroll-driven animations and
+view transitions for other tasks, and the ladder would have skipped them when
+the task didn't need them. Lookup, not license.