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/subagents/executor.py

@@ -13,7 +13,7 @@ from typing import Any
 
 from langchain.agents import create_agent
 from langchain.tools import BaseTool
-from langchain_core.messages import AIMessage, HumanMessage
+from langchain_core.messages import AIMessage, HumanMessage, SystemMessage
 from langchain_core.runnables import RunnableConfig
 
 from deerflow.agents.thread_state import SandboxState, ThreadDataState, ThreadState
@@ -184,7 +184,63 @@ class SubagentExecutor:
             state_schema=ThreadState,
         )
 
-    def _build_initial_state(self, task: str) -> dict[str, Any]:
+    async def _load_skill_messages(self) -> list[SystemMessage]:
+        """Load skill content as conversation items based on config.skills.
+
+        Aligned with Codex's pattern: each subagent loads its own skills
+        per-session and injects them as conversation items (developer messages),
+        not as system prompt text. The config.skills whitelist controls which
+        skills are loaded:
+        - None: load all enabled skills
+        - []: no skills
+        - ["skill-a", "skill-b"]: only these skills
+
+        Returns:
+            List of SystemMessages containing skill content.
+        """
+        if self.config.skills is not None and len(self.config.skills) == 0:
+            logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} skills=[] — skipping skill loading")
+            return []
+
+        try:
+            from deerflow.skills.loader import load_skills
+
+            # Use asyncio.to_thread to avoid blocking the event loop (LangGraph ASGI requirement)
+            all_skills = await asyncio.to_thread(load_skills, enabled_only=True)
+            logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} loaded {len(all_skills)} enabled skills from disk")
+        except Exception:
+            logger.warning(f"[trace={self.trace_id}] Failed to load skills for subagent {self.config.name}", exc_info=True)
+            return []
+
+        if not all_skills:
+            logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} no enabled skills found")
+            return []
+
+        # Filter by config.skills whitelist
+        if self.config.skills is not None:
+            allowed = set(self.config.skills)
+            skills = [s for s in all_skills if s.name in allowed]
+        else:
+            skills = all_skills
+
+        if not skills:
+            return []
+
+        # Read each skill's SKILL.md content and create conversation items
+        messages = []
+        for skill in skills:
+            try:
+                content = await asyncio.to_thread(skill.skill_file.read_text, encoding="utf-8")
+                content = content.strip()
+                if content:
+                    messages.append(SystemMessage(content=f'<skill name="{skill.name}">\n{content}\n</skill>'))
+                    logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} loaded skill: {skill.name}")
+            except Exception:
+                logger.debug(f"[trace={self.trace_id}] Failed to read skill {skill.name}", exc_info=True)
+
+        return messages
+
+    async def _build_initial_state(self, task: str) -> dict[str, Any]:
         """Build the initial state for agent execution.
 
         Args:
@@ -193,8 +249,17 @@ class SubagentExecutor:
         Returns:
             Initial state dictionary.
         """
+        # Load skills as conversation items (Codex pattern)
+        skill_messages = await self._load_skill_messages()
+
+        messages: list = []
+        # Skill content injected as developer/system messages before the task
+        messages.extend(skill_messages)
+        # Then the actual task
+        messages.append(HumanMessage(content=task))
+
         state: dict[str, Any] = {
-            "messages": [HumanMessage(content=task)],
+            "messages": messages,
         }
 
         # Pass through sandbox and thread data from parent
@@ -230,7 +295,7 @@ class SubagentExecutor:
 
         try:
             agent = self._create_agent()
-            state = self._build_initial_state(task)
+            state = await self._build_initial_state(task)
 
             # Build config with thread_id for sandbox access and recursion limit
             run_config: RunnableConfig = {