feat(subagents): support per-subagent skill loading and custom subagent types (#2253)

* feat(subagents): support per-subagent skill loading and custom subagent types (#2230)

Add per-subagent skill configuration and custom subagent type registration,
aligned with Codex's role-based config layering and per-session skill injection.

Backend:
- SubagentConfig gains `skills` field (None=all, []=none, list=whitelist)
- New CustomSubagentConfig for user-defined subagent types in config.yaml
- SubagentsAppConfig gains `custom_agents` section and `get_skills_for()`
- Registry resolves custom agents with three-layer config precedence
- SubagentExecutor loads skills per-session as conversation items (Codex pattern)
- task_tool no longer appends skills to system_prompt
- Lead agent system prompt dynamically lists all registered subagent types
- setup_agent tool accepts optional skills parameter
- Gateway agents API transparently passes skills in CRUD operations

Frontend:
- Agent/CreateAgentRequest/UpdateAgentRequest types include skills field
- Agent card displays skills as badges alongside tool_groups

Config:
- config.example.yaml documents custom_agents and per-agent skills override

Tests:
- 40 new tests covering all skill config, custom agents, and registry logic
- Existing tests updated for new get_skills_prompt_section signature

Closes #2230

* fix: address review feedback on skills PR

- Remove stale get_skills_prompt_section monkeypatches from test_task_tool_core_logic.py
  (task_tool no longer imports this function after skill injection moved to executor)
- Add key prefixes (tg:/sk:) to agent-card badges to prevent React key collisions
  between tool_groups and skills

* fix(ci): resolve lint and test failures

- Format agent-card.tsx with prettier (lint-frontend)
- Remove stale "Skills Appendix" system_prompt assertion — skills are now
  loaded per-session by SubagentExecutor, not appended to system_prompt

* fix(ci): sort imports in test_subagent_skills_config.py (ruff I001)

* fix(ci): use nullish coalescing in agent-card badge condition (eslint)

* fix: address review feedback on skills PR

- Use model_fields_set in AgentUpdateRequest to distinguish "field omitted"
  from "explicitly set to null" — fixes skills=None ambiguity where None
  means "inherit all" but was treated as "don't change"
- Move lazy import of get_subagent_config outside loop in
  _build_available_subagents_description to avoid repeated import overhead

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/packages/harness/deerflow/tools/builtins/setup_agent_tool.py

@@ -17,12 +17,14 @@ def setup_agent(
     soul: str,
     description: str,
     runtime: ToolRuntime,
+    skills: list[str] | None = None,
 ) -> Command:
     """Setup the custom DeerFlow agent.
 
     Args:
         soul: Full SOUL.md content defining the agent's personality and behavior.
         description: One-line description of what the agent does.
+        skills: Optional list of skill names this agent should use. None means use all enabled skills, empty list means no skills.
     """
 
     agent_name: str | None = runtime.context.get("agent_name") if runtime.context else None
@@ -41,6 +43,8 @@ def setup_agent(
             config_data: dict = {"name": agent_name}
             if description:
                 config_data["description"] = description
+            if skills is not None:
+                config_data["skills"] = skills
 
             config_file = agent_dir / "config.yaml"
             with open(config_file, "w", encoding="utf-8") as f: