core/config: default system prompt for coding agent tools

tests/test_config/test_agent_section.py

@@ -2,7 +2,7 @@ from __future__ import annotations
 
 from typing import TYPE_CHECKING
 
-from plyngent.config import load
+from plyngent.config import DEFAULT_SYSTEM_PROMPT, load
 
 if TYPE_CHECKING:
     from pathlib import Path
@@ -12,7 +12,8 @@ def test_agent_section_defaults(tmp_path: Path) -> None:
     path = tmp_path / "c.toml"
     _ = path.write_text("", encoding="utf-8")
     store = load(path)
-    assert store.agent_config.system_prompt == ""
+    assert store.agent_config.system_prompt == DEFAULT_SYSTEM_PROMPT
+    assert "professional coding agent" in store.agent_config.system_prompt
     assert store.agent_config.max_tool_result_chars == 32_000
     assert store.agent_config.parallel_tools is True
     assert store.agent_config.confirm_destructive is True
@@ -41,3 +42,16 @@ max_context_tokens = 5000
     assert store.agent_config.confirm_destructive is False
     assert store.agent_config.path_denylist == ["/secrets/", ".ssh/"]
     assert store.agent_config.max_context_tokens == 5000
+
+
+def test_agent_system_prompt_empty_disables(tmp_path: Path) -> None:
+    path = tmp_path / "c.toml"
+    _ = path.write_text(
+        """
+[agent]
+system_prompt = ""
+""",
+        encoding="utf-8",
+    )
+    store = load(path)
+    assert store.agent_config.system_prompt == ""