Add local logo assets, stop hotlinking vgi-rpc-python's site

This was the only repo in the vgi-rpc fleet with no local logo pipeline at
all — its README's logo <img> hotlinked another repo's deployed docs
domain (vgi-rpc-python.query.farm), a fragile cross-repo dependency (if
that site's layout or domain ever changes, this README's image silently
breaks).

Added assets/logo-master.png (the genuinely transparent master shared
across the fleet) and a derived assets/vgi-logo.png (600w, matching the
java/rust ports' own derived-asset convention), and pointed the README at
this repo's own raw.githubusercontent.com URL instead.

Not ported: the java/rust regenerate_logo_assets.py script's landing-page-
patching step, since this repo has no landing.html to patch (no HTTP demo
page). The two asset files above were generated directly from the shared
master with a one-off PIL resize instead.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CZjHueBJtuRsGCEF18SDCn

Author: rustyconover

README.md

@@ -1,5 +1,5 @@
 <p align="center">
-  <img src="https://vgi-rpc-python.query.farm/assets/logo-shield.png" alt="Vector Gateway Interface logo" width="280">
+  <img src="https://raw.githubusercontent.com/Query-farm/vgi-rpc-csharp/main/assets/vgi-logo.png" alt="Vector Gateway Interface logo" width="320">
 </p>
 
 <h1 align="center">vgi-rpc for .NET</h1>