feat(viz-platform): create plugin structure (#177)

- Add plugins/viz-platform/ directory structure
- Create .claude-plugin/plugin.json with metadata
- Create .mcp.json pointing to viz-platform MCP server
- Create hooks/hooks.json with SessionStart hook
- Create symlink to mcp-servers/viz-platform
- Add plugin to marketplace.json

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/viz-platform/hooks/hooks.json

@@ -0,0 +1,10 @@
+{
+  "hooks": [
+    {
+      "event": "SessionStart",
+      "type": "command",
+      "command": "echo 'viz-platform plugin loaded'",
+      "timeout": 5000
+    }
+  ]
+}