core/config: add [agent] section for profile defaults

Parse and sync system_prompt, max_tool_result_chars, and parallel_tools.

tests/test_config/test_agent_section.py

@@ -0,0 +1,34 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from plyngent.config import load
+
+if TYPE_CHECKING:
+    from pathlib import Path
+
+
+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.max_tool_result_chars == 32_000  # noqa: PLR2004
+    assert store.agent_config.parallel_tools is True
+
+
+def test_agent_section_parse(tmp_path: Path) -> None:
+    path = tmp_path / "c.toml"
+    _ = path.write_text(
+        """
+[agent]
+system_prompt = "Be brief."
+max_tool_result_chars = 100
+parallel_tools = false
+""",
+        encoding="utf-8",
+    )
+    store = load(path)
+    assert store.agent_config.system_prompt == "Be brief."
+    assert store.agent_config.max_tool_result_chars == 100  # noqa: PLR2004
+    assert store.agent_config.parallel_tools is False