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/agents/lead_agent/prompt.py

@@ -164,6 +164,36 @@ Skip simple one-off tasks.
 """
 
 
+def _build_available_subagents_description(available_names: list[str], bash_available: bool) -> str:
+    """Dynamically build subagent type descriptions from registry.
+
+    Mirrors Codex's pattern where agent_type_description is dynamically generated
+    from all registered roles, so the LLM knows about every available type.
+    """
+    # Built-in descriptions (kept for backward compatibility with existing prompt quality)
+    builtin_descriptions = {
+        "general-purpose": "For ANY non-trivial task - web research, code exploration, file operations, analysis, etc.",
+        "bash": (
+            "For command execution (git, build, test, deploy operations)" if bash_available else "Not available in the current sandbox configuration. Use direct file/web tools or switch to AioSandboxProvider for isolated shell access."
+        ),
+    }
+
+    # Lazy import moved outside loop to avoid repeated import overhead
+    from deerflow.subagents.registry import get_subagent_config
+
+    lines = []
+    for name in available_names:
+        if name in builtin_descriptions:
+            lines.append(f"- **{name}**: {builtin_descriptions[name]}")
+        else:
+            config = get_subagent_config(name)
+            if config is not None:
+                desc = config.description.split("\n")[0].strip()  # First line only for brevity
+                lines.append(f"- **{name}**: {desc}")
+
+    return "\n".join(lines)
+
+
 def _build_subagent_section(max_concurrent: int) -> str:
     """Build the subagent system prompt section with dynamic concurrency limit.
 
@@ -174,13 +204,12 @@ def _build_subagent_section(max_concurrent: int) -> str:
         Formatted subagent section string.
     """
     n = max_concurrent
-    bash_available = "bash" in get_available_subagent_names()
-    available_subagents = (
-        "- **general-purpose**: For ANY non-trivial task - web research, code exploration, file operations, analysis, etc.\n- **bash**: For command execution (git, build, test, deploy operations)"
-        if bash_available
-        else "- **general-purpose**: For ANY non-trivial task - web research, code exploration, file operations, analysis, etc.\n"
-        "- **bash**: Not available in the current sandbox configuration. Use direct file/web tools or switch to AioSandboxProvider for isolated shell access."
-    )
+    available_names = get_available_subagent_names()
+    bash_available = "bash" in available_names
+
+    # Dynamically build subagent type descriptions from registry (aligned with Codex's
+    # agent_type_description pattern where all registered roles are listed in the tool spec).
+    available_subagents = _build_available_subagents_description(available_names, bash_available)
     direct_tool_examples = "bash, ls, read_file, web_search, etc." if bash_available else "ls, read_file, web_search, etc."
     direct_execution_example = (
         '# User asks: "Run the tests"\n# Thinking: Cannot decompose into parallel sub-tasks\n# → Execute directly\n\nbash("npm test")  # Direct execution, not task()'