skills/config: add the [skills] section

Skills are discovered from user-level directories by default: an explicit
``paths`` list wins, then the built-in sources (our own user config dir, and
other harnesses' skill dirs) with the project-scope dirs behind their own
switch. A name found twice shadows the lower-priority copy without hiding it.

``inject_catalog`` / ``max_catalog_skills`` bound what reaches the system
prompt, ``enabled = false`` registers nothing, and ``allow_write`` is the
standing grant for editing skills (default: ask every time). A malformed
section degrades to the defaults, like [agent] and [mcp].

tests/test_config/test_skills_section.py

@@ -0,0 +1,69 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from plyngent.config import load
+
+if TYPE_CHECKING:
+    from pathlib import Path
+
+
+def test_skills_section_defaults(tmp_path: Path) -> None:
+    path = tmp_path / "c.toml"
+    _ = path.write_text("", encoding="utf-8")
+    store = load(path)
+    assert store.skills_config.enabled is True
+    assert store.skills_config.paths == []
+    assert store.skills_config.discover == ["plyngent", "claude"]
+    assert store.skills_config.include_project_roots is True
+    assert store.skills_config.inject_catalog is True
+    assert store.skills_config.max_catalog_skills == 50
+    assert store.skills_config.allow_write is False
+
+
+def test_skills_section_parse(tmp_path: Path) -> None:
+    path = tmp_path / "c.toml"
+    _ = path.write_text(
+        """
+[skills]
+enabled = false
+paths = ["~/team-skills", "/srv/skills"]
+discover = ["plyngent"]
+include_project_roots = false
+inject_catalog = false
+max_catalog_skills = 5
+allow_write = true
+""",
+        encoding="utf-8",
+    )
+    cfg = load(path).skills_config
+    assert cfg.enabled is False
+    assert cfg.paths == ["~/team-skills", "/srv/skills"]
+    assert cfg.discover == ["plyngent"]
+    assert cfg.include_project_roots is False
+    assert cfg.inject_catalog is False
+    assert cfg.max_catalog_skills == 5
+    assert cfg.allow_write is True
+
+
+def test_skills_section_invalid_falls_back(tmp_path: Path) -> None:
+    """An unknown discovery source or a bad type degrades to the defaults."""
+    path = tmp_path / "c.toml"
+    _ = path.write_text(
+        """
+[skills]
+discover = ["codex"]
+""",
+        encoding="utf-8",
+    )
+    assert load(path).skills_config.discover == ["plyngent", "claude"]
+
+    path2 = tmp_path / "d.toml"
+    _ = path2.write_text(
+        """
+[skills]
+enabled = "yes"
+""",
+        encoding="utf-8",
+    )
+    assert load(path2).skills_config.enabled is True