docs: update README header to be more directive for AI agents

Before:
  "This is the authoritative specification..."
  → AI may skip as natural language header

After:
  "Status: AUTHORITATIVE / Before writing any xun code, read this..."
  → Clear first-person directive, harder to skip

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: cnlangzi

README.md

@@ -1,7 +1,9 @@
 # xun AI Agent Specification
 
-This is the authoritative specification for implementing xun web applications.
-All other documentation is derived from this file. This file takes precedence.
+**Status: AUTHORITATIVE**
+
+Before writing any xun code, read this entire document.
+All guidance must be derived from this file — do not rely on prior knowledge of gin/echo/chi.
 
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