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.

cloud/db.py

@@ -944,6 +944,7 @@ POSTGRES_SCHEMA = (
     "coordinator_parts TEXT, "
     "parent_task_id TEXT, "
     "coordinator_part TEXT, "
+    "kind TEXT, "
     "goal_resume_pending INTEGER DEFAULT 0, "
     "agent_name TEXT, "
     "agent_version TEXT, "
@@ -954,6 +955,8 @@ POSTGRES_SCHEMA = (
     "audit_loop_packet TEXT, "
     "UNIQUE(project_id, idempotency_key))",
     "CREATE INDEX IF NOT EXISTS idx_tasks_status ON tasks(status)",
+    "CREATE INDEX IF NOT EXISTS idx_tasks_parent"
+    " ON tasks(parent_task_id)",
     # 附件包与任务附件：与 server._ensure_uploads_tables 同款，
     # POSTGRES_SCHEMA 在每次初始化时全量执行，新表自动补建。
     "CREATE TABLE IF NOT EXISTS uploads("