Fix PR diff documentation to use git instead of tea

The tea CLI doesn't have a command to output PR diff content directly.
The -f diff flag only returns a URL. Updated docs to use tea pulls
checkout followed by git diff main...HEAD.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

agents/code-reviewer/AGENT.md

@@ -13,7 +13,7 @@ You are a code review specialist that provides immediate, structured feedback on
 
 You will receive a PR number to review. Follow this process:
 
-1. Fetch PR diff using `tea pulls <number> -f diff`
+1. Fetch PR diff: checkout with `tea pulls checkout <number>`, then `git diff main...HEAD`
 2. Analyze the diff for issues in these categories:
    - **Code Quality**: Readability, maintainability, complexity
    - **Bugs**: Logic errors, edge cases, null checks