fix: correct plugin install instructions in README

Replace non-existent `/install-plugin` command with the actual
Claude Code CLI commands:
  claude plugin marketplace add treble-app/cli
  claude plugin install treble

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

Author: gold-ak47

README.md

@@ -41,10 +41,9 @@ The output is a real project — `npm run dev` works, components match the Figma
 
 ### 1. Install the Claude Code plugin
 
-In [Claude Code](https://claude.ai/code):
-
-```
-/install-plugin treble-app/cli
+```bash
+claude plugin marketplace add treble-app/cli
+claude plugin install treble
 ```
 
 ### 2. Install the CLI