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/config/subagents_config.py

@@ -25,6 +25,47 @@ class SubagentOverrideConfig(BaseModel):
         min_length=1,
         description="Model name for this subagent (None = inherit from parent agent)",
     )
+    skills: list[str] | None = Field(
+        default=None,
+        description="Skill names whitelist for this subagent (None = inherit all enabled skills, [] = no skills)",
+    )
+
+
+class CustomSubagentConfig(BaseModel):
+    """User-defined subagent type declared in config.yaml."""
+
+    description: str = Field(
+        description="When the lead agent should delegate to this subagent",
+    )
+    system_prompt: str = Field(
+        description="System prompt that guides the subagent's behavior",
+    )
+    tools: list[str] | None = Field(
+        default=None,
+        description="Tool names whitelist (None = inherit all tools from parent)",
+    )
+    disallowed_tools: list[str] | None = Field(
+        default_factory=lambda: ["task", "ask_clarification", "present_files"],
+        description="Tool names to deny",
+    )
+    skills: list[str] | None = Field(
+        default=None,
+        description="Skill names whitelist (None = inherit all enabled skills, [] = no skills)",
+    )
+    model: str = Field(
+        default="inherit",
+        description="Model to use - 'inherit' uses parent's model",
+    )
+    max_turns: int = Field(
+        default=50,
+        ge=1,
+        description="Maximum number of agent turns before stopping",
+    )
+    timeout_seconds: int = Field(
+        default=900,
+        ge=1,
+        description="Maximum execution time in seconds",
+    )
 
 
 class SubagentsAppConfig(BaseModel):
@@ -44,6 +85,10 @@ class SubagentsAppConfig(BaseModel):
         default_factory=dict,
         description="Per-agent configuration overrides keyed by agent name",
     )
+    custom_agents: dict[str, CustomSubagentConfig] = Field(
+        default_factory=dict,
+        description="User-defined subagent types keyed by agent name",
+    )
 
     def get_timeout_for(self, agent_name: str) -> int:
         """Get the effective timeout for a specific agent.
@@ -82,6 +127,20 @@ class SubagentsAppConfig(BaseModel):
             return self.max_turns
         return builtin_default
 
+    def get_skills_for(self, agent_name: str) -> list[str] | None:
+        """Get the skills override for a specific agent.
+
+        Args:
+            agent_name: The name of the subagent.
+
+        Returns:
+            Skill names whitelist if overridden, None otherwise (subagent will inherit all enabled skills).
+        """
+        override = self.agents.get(agent_name)
+        if override is not None and override.skills is not None:
+            return override.skills
+        return None
+
 
 _subagents_config: SubagentsAppConfig = SubagentsAppConfig()
 
@@ -105,15 +164,20 @@ def load_subagents_config_from_dict(config_dict: dict) -> None:
             parts.append(f"max_turns={override.max_turns}")
         if override.model is not None:
             parts.append(f"model={override.model}")
+        if override.skills is not None:
+            parts.append(f"skills={override.skills}")
         if parts:
             overrides_summary[name] = ", ".join(parts)
 
-    if overrides_summary:
+    custom_agents_names = list(_subagents_config.custom_agents.keys())
+
+    if overrides_summary or custom_agents_names:
         logger.info(
-            "Subagents config loaded: default timeout=%ss, default max_turns=%s, per-agent overrides=%s",
+            "Subagents config loaded: default timeout=%ss, default max_turns=%s, per-agent overrides=%s, custom_agents=%s",
             _subagents_config.timeout_seconds,
             _subagents_config.max_turns,
-            overrides_summary,
+            overrides_summary or "none",
+            custom_agents_names or "none",
         )
     else:
         logger.info(