fix(sisyphus-agent): add GitHub markdown rules to prevent broken code block rendering

The change adds a new "GitHub Markdown Rules" section to the sisyphus agent prompt that specifies:
- Code blocks MUST have exactly 3 backticks with language identifier
- Every opening ``` MUST have a matching closing ``` on its own line
- No trailing backticks or spaces after closing ```
- Inline code should use single backticks
- Lists inside code blocks break rendering

This fixes the issue where code blocks in GitHub comments weren't being closed properly, causing broken markdown rendering.

🤖 Generated with assistance of [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

.github/workflows/sisyphus-agent.yml

@@ -186,6 +186,25 @@ jobs:
           )"
           ```
 
+          ### GitHub Markdown Rules (MUST FOLLOW)
+
+          **Code blocks MUST have EXACTLY 3 backticks and language identifier:**
+          - CORRECT: ` ```bash ` ... ` ``` `
+          - WRONG: ` ``` ` (no language), ` ```` ` (4 backticks), ` `` ` (2 backticks)
+          
+          **Every opening ` ``` ` MUST have a closing ` ``` ` on its own line:**
+          ```
+          ```bash
+          code here
+          ```
+          ```
+          
+          **NO trailing backticks or spaces after closing ` ``` `**
+          
+          **For inline code, use SINGLE backticks:** `code` not ```code```
+          
+          **Lists inside code blocks break rendering - avoid them or use plain text**
+
           ### Rules
           - EVERY response = GitHub comment (use heredoc for proper escaping)
           - Code changes = PR (never push main/master)