feat(skills): support recursive nested skill loading (#950)

* feat(skills): support recursive nested skill loading

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

backend/tests/test_skills_loader.py

@@ -0,0 +1,57 @@
+"""Tests for recursive skills loading."""
+
+from pathlib import Path
+
+from src.skills.loader import load_skills
+
+
+def _write_skill(skill_dir: Path, name: str, description: str) -> None:
+    """Write a minimal SKILL.md for tests."""
+    skill_dir.mkdir(parents=True, exist_ok=True)
+    content = f"---\nname: {name}\ndescription: {description}\n---\n\n# {name}\n"
+    (skill_dir / "SKILL.md").write_text(content, encoding="utf-8")
+
+
+def test_load_skills_discovers_nested_skills_and_sets_container_paths(tmp_path: Path):
+    """Nested skills should be discovered recursively with correct container paths."""
+    skills_root = tmp_path / "skills"
+
+    _write_skill(skills_root / "public" / "root-skill", "root-skill", "Root skill")
+    _write_skill(skills_root / "public" / "parent" / "child-skill", "child-skill", "Child skill")
+    _write_skill(skills_root / "custom" / "team" / "helper", "team-helper", "Team helper")
+
+    skills = load_skills(skills_path=skills_root, use_config=False, enabled_only=False)
+    by_name = {skill.name: skill for skill in skills}
+
+    assert {"root-skill", "child-skill", "team-helper"} <= set(by_name)
+
+    root_skill = by_name["root-skill"]
+    child_skill = by_name["child-skill"]
+    team_skill = by_name["team-helper"]
+
+    assert root_skill.skill_path == "root-skill"
+    assert root_skill.get_container_file_path() == "/mnt/skills/public/root-skill/SKILL.md"
+
+    assert child_skill.skill_path == "parent/child-skill"
+    assert child_skill.get_container_file_path() == "/mnt/skills/public/parent/child-skill/SKILL.md"
+
+    assert team_skill.skill_path == "team/helper"
+    assert team_skill.get_container_file_path() == "/mnt/skills/custom/team/helper/SKILL.md"
+
+
+def test_load_skills_skips_hidden_directories(tmp_path: Path):
+    """Hidden directories should be excluded from recursive discovery."""
+    skills_root = tmp_path / "skills"
+
+    _write_skill(skills_root / "public" / "visible" / "ok-skill", "ok-skill", "Visible skill")
+    _write_skill(
+        skills_root / "public" / "visible" / ".hidden" / "secret-skill",
+        "secret-skill",
+        "Hidden skill",
+    )
+
+    skills = load_skills(skills_path=skills_root, use_config=False, enabled_only=False)
+    names = {skill.name for skill in skills}
+
+    assert "ok-skill" in names
+    assert "secret-skill" not in names