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>

frontend/src/components/workspace/agents/agent-card.tsx

@@ -79,14 +79,27 @@ export function AgentCard({ agent }: AgentCardProps) {
           )}
         </CardHeader>
 
-        {agent.tool_groups && agent.tool_groups.length > 0 && (
+        {(agent.tool_groups?.length ?? agent.skills?.length ?? 0) > 0 && (
           <CardContent className="pt-0 pb-3">
             <div className="flex flex-wrap gap-1">
-              {agent.tool_groups.map((group) => (
-                <Badge key={group} variant="outline" className="text-xs">
+              {agent.tool_groups?.map((group) => (
+                <Badge
+                  key={`tg:${group}`}
+                  variant="outline"
+                  className="text-xs"
+                >
                   {group}
                 </Badge>
               ))}
+              {agent.skills?.map((skill) => (
+                <Badge
+                  key={`sk:${skill}`}
+                  variant="secondary"
+                  className="text-xs"
+                >
+                  {skill}
+                </Badge>
+              ))}
             </div>
           </CardContent>
         )}

frontend/src/core/agents/types.ts

@@ -3,6 +3,7 @@ export interface Agent {
   description: string;
   model: string | null;
   tool_groups: string[] | null;
+  skills: string[] | null;
   soul?: string | null;
 }
 
@@ -11,6 +12,7 @@ export interface CreateAgentRequest {
   description?: string;
   model?: string | null;
   tool_groups?: string[] | null;
+  skills?: string[] | null;
   soul?: string;
 }
 
@@ -18,5 +20,6 @@ export interface UpdateAgentRequest {
   description?: string | null;
   model?: string | null;
   tool_groups?: string[] | null;
+  skills?: string[] | null;
   soul?: string | null;
 }