docs: clarify that simplicity applies to user's app, not the harness (#754)

The previous wording "Keep it simple and reliable" could be misread as
claiming Perstack itself is simple. The intent is that Perstack absorbs
harness complexity so the user's agentic app stays simple and reliable.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: FL4TLiN3

README.md

@@ -21,7 +21,7 @@ Perstack is a harness for agentic apps. It aims to:
 
 - **Do big things with small models**: If a smaller model can do the job, there's no reason to use a bigger one.
 - **Quality is a system property, not a model property**: Building agentic software people actually use doesn't require an AI science degree—just a solid understanding of the problems you're solving.
-- **Keep it simple and reliable**: The biggest mistake is cramming AI into an overly complex harness and ending up with an inoperable agent.
+- **Keep your app simple and reliable**: The harness is inevitably complex—Perstack absorbs that complexity so your agentic app doesn't have to.
 
 ## Getting Started