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/tests/test_p2_migrations.py

@@ -240,6 +240,48 @@ class TestMigrateNewTables(unittest.TestCase):
                     "result_text", "error_text", "created_at"):
             self.assertIn(col, attempts_inserts[0])
 
+    def test_kind_column_backfilled_when_missing(self):
+        # P2：PG 侧 tasks 尚无 kind 列时，迁移必须先 ALTER 补列
+        # 再直插，且插入列清单里含 kind；kind 已存在时不重复
+        # ALTER（与 agent_* 列同款 backfill 惯例）。
+        with tempfile.TemporaryDirectory() as tmp:
+            src = os.path.join(tmp, "src.db")
+            _make_sqlite(src)
+            fake = FakePgConn(pragma_columns={
+                "projects": {"repo_url"},
+                "tasks": {"id", "project_id"},
+                "task_followups": {"id", "task_id", "project_id",
+                                   "body", "mode", "status",
+                                   "created_by", "sender_task_id",
+                                   "created_at", "updated_at"},
+            })
+            result = migrate_sqlite_to_postgres(src, pg_conn=fake)
+        self.assertEqual(result.get("tasks"), 1, "tasks")
+        self.assertTrue(
+            any("tasks" in a and "kind" in a for a in fake.alters),
+            "未发出 tasks.kind 补列 ALTER：%s" % fake.alters)
+        # 注：插入列清单取自源 SQLite 表的列，夹具源表无 kind
+        # 列故清单里无 kind；生产源库（init_db 建表）有 kind，
+        # 迁移时按列原样搬运（与 agent_* 列同款）。
+
+    def test_kind_column_present_skips_alter(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            src = os.path.join(tmp, "src.db")
+            _make_sqlite(src)
+            fake = FakePgConn(pragma_columns={
+                "projects": {"repo_url"},
+                "tasks": {"id", "project_id", "kind"},
+                "task_followups": {"id", "task_id", "project_id",
+                                   "body", "mode", "status",
+                                   "created_by", "sender_task_id",
+                                   "created_at", "updated_at"},
+            })
+            result = migrate_sqlite_to_postgres(src, pg_conn=fake)
+        self.assertEqual(result.get("tasks"), 1, "tasks")
+        self.assertFalse(
+            any("tasks" in a and "kind" in a for a in fake.alters),
+            "kind 已存在却仍发补列 ALTER：%s" % fake.alters)
+
     def test_sender_column_present_skips_alter(self):
         with tempfile.TemporaryDirectory() as tmp:
             src = os.path.join(tmp, "src.db")