blog: announce OpenAI Codex for Open Source grant (/blog/29) (#3003)

Add a blog post for the OpenAI Codex for Open Source grant, list it on
the blog index, and repoint the OpenAI sponsor logo (README + landing) to
the post — matching how the Anthropic (/blog/3) and Atlas Cloud (/blog/8)
sponsor logos link to their posts.

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

README.md

@@ -8,7 +8,7 @@ Build an agent and it gets a model, memory, and tools, manages your services, an
 
 <a href="https://go-micro.dev/blog/3"><img src="https://upload.wikimedia.org/wikipedia/commons/7/78/Anthropic_logo.svg" height="26" /></a>
 &nbsp;&nbsp;
-<a href="https://openai.com"><img src="https://upload.wikimedia.org/wikipedia/commons/4/4d/OpenAI_Logo.svg" height="26" /></a>
+<a href="https://go-micro.dev/blog/29"><img src="https://upload.wikimedia.org/wikipedia/commons/4/4d/OpenAI_Logo.svg" height="26" /></a>
 &nbsp;&nbsp;
 <a href="https://go-micro.dev/blog/8"><img src="https://www.atlascloud.ai/logo.svg" height="26" /></a>