docs: Add upgrade instructions to READMEs

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: snoo

README.ko.md

@@ -44,6 +44,12 @@ codesyncer init
 "CLAUDE.md 읽어줘"
 ```
 
+### 업그레이드
+
+```bash
+npm install -g codesyncer@latest
+```
+
 ---
 
 ## 핵심 기능

README.md

@@ -44,6 +44,12 @@ codesyncer init
 "Read CLAUDE.md"
 ```
 
+### Upgrade
+
+```bash
+npm install -g codesyncer@latest
+```
+
 ---
 
 ## Core Features