docs(readme): swap square logo for horizontal wordmark banner

- README header now uses /static/brand/logo-wordmark.png (1200x364
  horizontal lockup) instead of the 120-wide square logo. The banner
  is wrapped in a link to agentshare.dev so it's clickable in GitHub.
- Adds ?v=2026-05-12 cache buster to bypass GitHub's
  camo.githubusercontent proxy cache for the previous square asset.

Wordmark is a content-bbox crop of the master Concept A lockup,
generated centrally in agent-price-api (commit 645308f).

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: anhmtk

README.md

@@ -1,5 +1,7 @@
 <p align="center">
-  <img src="https://agentshare.dev/static/logo.png" alt="AgentShare" width="120">
+  <a href="https://agentshare.dev">
+    <img src="https://agentshare.dev/static/brand/logo-wordmark.png?v=2026-05-12" alt="AgentShare — Price API + MCP for AI agents" width="520">
+  </a>
 </p>
 
 <h1 align="center">AgentShare MCP Server</h1>