feat(agent): add custom-agent self-updates with user isolation (#2713)

* feat(agent): add update_agent tool for in-chat custom-agent self-updates (#2616)

Custom agents had no built-in way to persist updates to their own SOUL.md /
config.yaml from a normal chat — `setup_agent` was only bound during the
bootstrap flow, so when the user asked the agent to refine its description
or personality, the agent would shell out via bash/write_file and the edits
landed in a temporary sandbox/tool workspace instead of
`{base_dir}/agents/{agent_name}/`.

Changes:
- New `update_agent` builtin tool with partial-update semantics (only the
  fields you pass are written) and atomic temp-file + os.replace writes so
  a failed update never corrupts existing SOUL.md / config.yaml.
- Lead agent now binds `update_agent` in the non-bootstrap path whenever
  `agent_name` is set in the runtime context. Default agent (no
  agent_name) and bootstrap flow are unchanged.
- New `<self_update>` system-prompt section is injected for custom agents,
  instructing them to use `update_agent` — and explicitly NOT bash /
  write_file — to persist self-updates.
- Tests: 11 new cases in `tests/test_update_agent_tool.py` covering
  validation (missing/invalid agent_name, unknown agent, no fields),
  partial updates (soul-only, description-only, skills=[] vs omitted),
  no-op detection, atomic-write safety, and AgentConfig round-tripping;
  plus 2 new cases in `tests/test_lead_agent_prompt.py` covering the
  self-update prompt section.
- Docs: updated backend/CLAUDE.md builtin tools list and tools.mdx
  (en/zh) with the new tool description.

* feat(agent): isolate custom agents per user

Store custom agent definitions under the effective user, keep legacy agents readable until migration, and cover API/tool/migration behavior with tests.

Co-authored-by: Cursor <cursoragent@cursor.com>

* feat: consistent write/delete targets & add --user-id to migration

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/content/en/harness/tools.mdx

@@ -64,6 +64,12 @@ Dynamically configures the current agent session. Used during the bootstrap flow
 
 ---
 
+### update_agent
+
+Persists updates to the current custom agent's `SOUL.md` and `config.yaml`. Bound to the lead agent only when a custom agent is active (`agent_name` is set in the runtime context). Use this when the user asks the agent to refine its own description, personality, skill whitelist, tool-group whitelist, or default model — it writes directly into the per-user layout `{base_dir}/users/{user_id}/agents/{agent_name}/`, so the change is picked up automatically on the next user turn. Only the fields you explicitly pass are updated; omit a field to preserve its existing value. Pass `skills=[]` to disable all skills, or omit `skills` to keep the existing whitelist.
+
+---
+
 ### invoke_acp_agent
 
 Invokes an external agent using the [Agent Connect Protocol (ACP)](https://agentconnectprotocol.org/). Requires `acp_agents:` configuration in `config.yaml`. See the [Subagents](/docs/harness/subagents) page for ACP configuration.

frontend/src/content/zh/harness/tools.mdx

@@ -61,6 +61,12 @@ task(agent="general-purpose", task="...", context="...")
 
 ---
 
+### update_agent
+
+将更新持久化到当前自定义 Agent 的 `SOUL.md` 和 `config.yaml`。仅当激活了自定义 Agent（运行时上下文中存在 `agent_name`）时，才会绑定到 lead agent。当用户在 Agent 内开启 chat 并要求该 Agent 调整自身的描述、人格、技能白名单、工具组白名单或默认模型时使用——它会直接写入按用户隔离的 `{base_dir}/users/{user_id}/agents/{agent_name}/` 下的真实配置文件，下一轮对话即可生效。仅显式传入的字段会被更新；省略某个字段以保留其现有值。传入 `skills=[]` 可禁用全部技能，省略 `skills` 则保留现有白名单。
+
+---
+
 ### invoke_acp_agent
 
 使用 [Agent Connect Protocol (ACP)](https://agentconnectprotocol.org/) 调用外部 Agent。需要在 `config.yaml` 中配置 `acp_agents:`。参见[子 Agent](/docs/harness/subagents)页面了解 ACP 配置。