docs: "The four pillars" → "Core capabilities" 헤더 변경

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

Author: thrcle

README.md

@@ -31,7 +31,7 @@ interface, not the identity** — Slack/Web are adapters on the same core.
 
 ---
 
-## The four pillars
+## Core capabilities
 
 | Pillar | What it is |
 |---|---|