Agent cards: shareable brain QR on the flip side

- Backend brainhub: AgentBrain gains resourceUrl
  (https://clawbrainhub.com/brains/{owner}/{name}); set in parse_brain + fake.
- Mobile: AgentMeta gains brainUrl (carried from the pulled brain's resourceUrl).
- AgentBackTemplate: under Special capabilities, a "Share brain link" toggle
  reveals a QR (+ the URL) that anyone can scan to open the brain on
  ClawBrainHub.
- Gate green: mobile 28, backend brainhub tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

cardclaws-mobile/app/demo.tsx

@@ -193,6 +193,7 @@ export default function CardEditorScreen() {
           skills: b.skills.map((s) => ({ name: s, level: 4 })),
           tools: b.tools,
           capabilities: b.capabilities,
+          brainUrl: b.resourceUrl,
         });
         setShowing(false);
         draft.setPendingAgentBrain(null);