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/task_tool.py

@@ -10,7 +10,6 @@ from langchain.tools import InjectedToolCallId, ToolRuntime, tool
 from langgraph.config import get_stream_writer
 from langgraph.typing import ContextT
 
-from deerflow.agents.lead_agent.prompt import get_skills_prompt_section
 from deerflow.agents.thread_state import ThreadState
 from deerflow.sandbox.security import LOCAL_BASH_SUBAGENT_DISABLED_MESSAGE, is_host_bash_allowed
 from deerflow.subagents import SubagentExecutor, get_available_subagent_names, get_subagent_config
@@ -35,7 +34,7 @@ async def task_tool(
     - Handle complex multi-step tasks autonomously
     - Execute commands or operations in isolated contexts
 
-    Available subagent types depend on the active sandbox configuration:
+    Built-in subagent types:
     - **general-purpose**: A capable agent for complex, multi-step tasks that require
       both exploration and action. Use when the task requires complex reasoning,
       multiple dependent steps, or would benefit from isolated context.
@@ -43,6 +42,11 @@ async def task_tool(
       available when host bash is explicitly allowed or when using an isolated shell
       sandbox such as `AioSandboxProvider`.
 
+    Additional custom subagent types may be defined in config.yaml under
+    `subagents.custom_agents`. Each custom type can have its own system prompt,
+    tools, skills, model, and timeout configuration. If an unknown subagent_type
+    is provided, the error message will list all available types.
+
     When to use this tool:
     - Complex tasks requiring multiple steps or tools
     - Tasks that produce verbose output
@@ -72,9 +76,9 @@ async def task_tool(
     # Build config overrides
     overrides: dict = {}
 
-    skills_section = get_skills_prompt_section()
-    if skills_section:
-        overrides["system_prompt"] = config.system_prompt + "\n\n" + skills_section
+    # Skills are loaded by SubagentExecutor per-session (aligned with Codex's pattern:
+    # each subagent loads its own skills based on config, injected as conversation items).
+    # No longer appended to system_prompt here.
 
     if max_turns is not None:
         overrides["max_turns"] = max_turns