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>

config.example.yaml

@@ -484,6 +484,12 @@ skills:
   # Default: /mnt/skills
   container_path: /mnt/skills
 
+# Note: To restrict which skills are loaded for a specific custom agent,
+# define a `skills` list in that agent's `config.yaml` (e.g. `agents/my-agent/config.yaml`):
+# - Omitted or null: load all globally enabled skills (default)
+# - []: disable all skills for this agent
+# - ["skill-name"]: load only specific skills
+
 # ============================================================================
 # Title Generation Configuration
 # ============================================================================