feat: side-thread task kind (Muse role alignment phase 2)

Tasks gain kind='main'|'side' (NULL=main). A side thread hangs under a
parent task, defaults to the local executor, never builds a worktree,
branch or container unless explicitly asked, and writes its conclusion
back to the parent via task_followups on terminal transition.

- server: kind validation + side-thread gates (parent required, no
  coordinator_parts/upload_id/environment_id, parent must not be a
  coordinator), local executor default, GET /tasks kind filter,
  /worktree 404 and /fork /merge /pr 409 for side threads.
- worker: side-default shunt in _run_attempt (no-worktree scratch cwd,
  in-memory plan=1 when a repo is present, seed/checkpoint skipped),
  _writeback_side_thread, marker task-create kind field + inheritance,
  side threads excluded from marker spawn counts.
- host/sandbox: --no-worktree plumbing for dispatch/goal/popen
  (Local + Docker), scratch cwd work/side-threads/<task-id>.
- console (classic only): kind filter + [旁聊] rows + '开旁聊' entry,
  side-thread dispatch mode, side-thread list in detail, merge/PR/fork
  hidden for side threads; build exit 0, console.html sha df39db5a.
- tests: new test_sidethread.py (29 tests) + kind backfill tests in
  test_p2_migrations.py; full cloud suite 3900 tests OK (skipped=13).

Design: verification/codex-muse-role-alignment-design-20261108.md
Summary: verification/codex-muse-role-align-p1p2-20261108.md
Local only: not pushed, not merged, not deployed.

docs/codex-native-ui-alignment.md

@@ -56,6 +56,7 @@
 | B16 | worker / 监控 / 模型（`GET /workers`、`/workers/heartbeat`、`/metrics`、`/metrics-report`、`/audit-scorecard`、`/routing-shadow`、`/models`、`/relay/models`） | `model/list`、`modelProvider/capabilities/read` 覆盖模型面；其余无 | 部分映射：模型列表有对应；worker 池、指标、评分卡、路由影子 **原生无对应表面** |
 | B17 | 待办（5 个端点，`/projects/{id}/todos`） | 无 | 原生无对应表面 |
 | B18 | 模型总览与中转模型列表（`GET /models`、`GET /relay/models`） | `model/list`（C 实测仅一条 `gpt-6.1-sol`） | 部分映射：单模型列表有对应；中转站多模型下拉无原生面（旧控制台用 `/relay/models` 做模型下拉，新 UI 用 `model/list`） |
+| B19 | 旁聊会话列表（`kind='side'` 任务：`POST /tasks` 带 `kind` + `parent_task_id`、`GET /tasks?kind=side`、`GET /tasks?parent_task_id=&kind=side`） | 无。原生 UI 只有线程列表，无“主聊 + 旁聊”分组、无 kind 筛选、无“开旁聊”入口 | 原生无对应表面（旁聊会话列表仅 `/console-classic`） |
 
 ## §3 旧 console.html 的去留决定