core/config: default system prompt for coding agent tools

doc/plyngent.example.toml

@@ -11,7 +11,12 @@
 # # url = ":memory:"
 
 [agent]
-system_prompt = "You are a careful coding assistant. Prefer small, verified edits."
+# system_prompt defaults to the built-in coding-agent guide when omitted.
+# Override with a multi-line literal (prefer ''' so nested " is fine):
+# system_prompt = '''
+# Your custom system prompt...
+# '''
+# system_prompt = ""   # disable system prompt entirely
 max_tool_result_chars = 32000
 parallel_tools = true
 confirm_destructive = true