feat/per agent skill filter (#1650)

* feat(agent): 为AgentConfig添加skills字段并更新lead_agent系统提示

在AgentConfig中添加skills字段以支持配置agent可用技能
更新lead_agent的系统提示模板以包含可用技能信息

* fix: resolve agent skill configuration edge cases and add tests

* Update backend/packages/harness/deerflow/agents/lead_agent/prompt.py

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

* refactor(agent): address PR review comments for skills configuration

- Add detailed docstring to `skills` field in `AgentConfig` to clarify the semantics of `None` vs `[]`.
- Add unit tests in `test_custom_agent.py` to verify `load_agent_config()` correctly parses omitted skills and explicit empty lists.
- Fix `test_make_lead_agent_empty_skills_passed_correctly` to include `agent_name` in the runtime config, ensuring it exercises the real code path.

* docs: 添加关于按代理过滤技能的配置说明

在配置示例文件和文档中添加说明，解释如何通过代理的config.yaml文件限制加载的技能

---------

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

backend/tests/test_custom_agent.py

@@ -164,6 +164,28 @@ class TestLoadAgentConfig:
 
         assert cfg.tool_groups == ["file:read", "file:write"]
 
+    def test_load_config_with_skills_empty_list(self, tmp_path):
+        config_dict = {"name": "no-skills-agent", "skills": []}
+        _write_agent(tmp_path, "no-skills-agent", config_dict)
+
+        with patch("deerflow.config.agents_config.get_paths", return_value=_make_paths(tmp_path)):
+            from deerflow.config.agents_config import load_agent_config
+
+            cfg = load_agent_config("no-skills-agent")
+
+        assert cfg.skills == []
+
+    def test_load_config_with_skills_omitted(self, tmp_path):
+        config_dict = {"name": "default-skills-agent"}
+        _write_agent(tmp_path, "default-skills-agent", config_dict)
+
+        with patch("deerflow.config.agents_config.get_paths", return_value=_make_paths(tmp_path)):
+            from deerflow.config.agents_config import load_agent_config
+
+            cfg = load_agent_config("default-skills-agent")
+
+        assert cfg.skills is None
+
     def test_legacy_prompt_file_field_ignored(self, tmp_path):
         """Unknown fields like the old prompt_file should be silently ignored."""
         agent_dir = tmp_path / "agents" / "legacy-agent"