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>

config.example.yaml

@@ -577,15 +577,41 @@ sandbox:
 #   # Optional global max-turn override for all subagents
 #   # max_turns: 120
 #
-#   # Optional per-agent overrides
+#   # Optional per-agent overrides (applies to both built-in and custom agents)
 #   agents:
 #     general-purpose:
 #       timeout_seconds: 1800  # 30 minutes for complex multi-step tasks
 #       max_turns: 160
 #       # model: qwen3:32b     # Use a specific model (default: inherit from lead agent)
+#       # skills:               # Skill whitelist (default: inherit all enabled skills)
+#       #   - web-search
+#       #   - data-analysis
 #     bash:
 #       timeout_seconds: 300   # 5 minutes for quick command execution
 #       max_turns: 80
+#       # skills: []            # No skills for bash agent
+#
+#   # Custom subagent types: define specialized agents with their own prompts,
+#   # tools, skills, and model configuration. Custom agents are available via
+#   # the `task` tool alongside built-in types (general-purpose, bash).
+#   # custom_agents:
+#   #   analysis:
+#   #     description: "Data analysis specialist for processing datasets and generating insights"
+#   #     system_prompt: |
+#   #       You are a data analysis subagent. Focus on:
+#   #       - Processing and analyzing datasets
+#   #       - Generating visualizations
+#   #       - Providing statistical insights
+#   #     tools:                  # Tool whitelist (null = inherit all)
+#   #       - bash
+#   #       - read_file
+#   #       - write_file
+#   #     skills:                 # Skill whitelist (null = inherit all, [] = none)
+#   #       - data-analysis
+#   #       - visualization
+#   #     model: inherit          # 'inherit' uses parent's model
+#   #     max_turns: 80
+#   #     timeout_seconds: 600
 #
 #   # Model override: by default, subagents inherit the lead agent's model.
 #   # Set `model` to use a different model (e.g., a local Ollama model for cost savings).