refactor(server): steward 意图改用声明式注册表编排

- 新增 steward_intent_registry，IntentDescriptor 统一描述意图的识别关键词、动作步骤构建、字段白名单与副作用集合，替代分散的 if/else
- 新增 steward_intent_bootstrap 注册 expense_application 等意图；新增 steward_query_executors 提供差旅标准查询的无副作用执行与城市/席别标签化输出
- action_contracts/action_executor/graph_planner/intent_agent/model_plan_builder/planner_extraction/fallback 适配注册表，识别与执行分发自动从注册表取数
- 新增 intent_registry/query_executors 测试，更新 intent_agent 测试

server/src/app/schemas/steward.py

@@ -4,8 +4,8 @@ from typing import Any, Literal
 
 from pydantic import BaseModel, Field
 
-StewardTaskType = Literal["expense_application", "reimbursement"]
-StewardAssignedAgent = Literal["application_assistant", "reimbursement_assistant"]
+StewardTaskType = str
+StewardAssignedAgent = str
 StewardPlanningSource = Literal["llm_function_call", "rule_fallback"]
 StewardPlanNextAction = Literal["confirm_flow", "confirm_task", "delegate_task", "none"]
 StewardRequestedAction = Literal["preview", "save_draft", "submit"]
@@ -22,20 +22,7 @@ StewardRuntimeNextAction = Literal[
     "cancel_current_action",
     "no_op",
 ]
-StewardActionType = Literal[
-    "detect_intent",
-    "fill_application_fields",
-    "build_application_preview",
-    "fill_reimbursement_fields",
-    "build_reimbursement_preview",
-    "validate_required_fields",
-    "run_duplicate_precheck",
-    "save_application_draft",
-    "submit_application",
-    "link_existing_application",
-    "create_reimbursement_draft",
-    "associate_attachments",
-]
+StewardActionType = str
 StewardActionStatus = Literal["completed", "planned", "pending_confirmation", "blocked"]
 StewardActionExecutionStatus = Literal["succeeded", "blocked", "needs_confirmation", "failed"]
 StewardTaskStatus = Literal[
@@ -47,7 +34,7 @@ StewardTaskStatus = Literal[
     "blocked",
 ]
 StewardConfirmationStatus = Literal["pending", "confirmed", "rejected"]
-StewardFlowId = Literal["travel_application", "travel_reimbursement"]
+StewardFlowId = str
 StewardPendingFlowStatus = Literal["none", "pending", "confirmed", "rejected"]