docs: Improve AI auto-adjust prompt with variable substitution guidance

Updated the default prompt template to explain how to use variables:
- Variables must be wrapped in curly braces: {variable}
- Use triple quotes since content may contain quotes/newlines
- Added example: my_var = """{response}"""

This helps the AI generate code that works correctly with the
variable substitution system.

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

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

src/smarttools/ui_urwid.py

@@ -1831,7 +1831,10 @@ class SmartToolsUI:
         # Default prompt template for AI code generation/adjustment
         default_ai_prompt = f"""Write inline Python code (NOT a function definition) according to my instruction.
 
-The code runs directly - variables are already available, no function needed.
+The code runs directly with variable substitution. To use an available variable:
+- Wrap the variable name in curly braces: {{variable}}
+- Use triple quotes since the substituted content may contain quotes/newlines
+- Example: my_var = \"\"\"{{response}}\"\"\"
 
 INSTRUCTION: [Describe what you want]