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/packages/harness/deerflow/agents/lead_agent/prompt.py

@@ -402,6 +402,10 @@ def get_skills_prompt_section(available_skills: set[str] | None = None) -> str:
     if available_skills is not None:
         skills = [skill for skill in skills if skill.name in available_skills]
 
+    # Check again after filtering
+    if not skills:
+        return ""
+
     skill_items = "\n".join(
         f"    <skill>\n        <name>{skill.name}</name>\n        <description>{skill.description}</description>\n        <location>{skill.get_container_file_path(container_base_path)}</location>\n    </skill>" for skill in skills
     )