feat: 提示词新增全局变量 DataDir + 触发合并/种子意图默认值调整

- prompt: 新增全局运行时变量 {{.DataDir}}(服务端数据根目录 s.m.dir),
  与 {{.Now}} 一样对所有 agent(planner/worker/mainagent/goals/chat 及自定义)可用;
  各 Vars 结构体加 DataDir 字段并在渲染时填入根 workDir,globalPromptVars 登记以便
  校验/UI 变量列表/预览识别(纯运行时变量,不入库,无需迁移)。
- trigger_merge_mode 默认 by_task→all(schema/config/后端兜底/前端初值同步)。
- seed_first_intent 默认 true→false(走标准先规划再执行);spawn_task 不再暴露该参数,
  工具建任务一律 false。

agent/prompt.go

@@ -15,10 +15,10 @@ var PromptOverride func(agentKey string) (string, bool)
 // Prompt-variable structs — fields mirror each agent's catalog (docs §5a) so a
 // user template referencing a catalog variable renders; referencing anything else
 // fails template execution and falls back to the built-in default.
-type PlannerVars struct{ Goal, Scope, AssetSummary, Now string }
-type WorkerVars struct{ ProxyAddr, WorkerName, Now string }
-type MainVars struct{ Goal, AssetSummary, FindingsSummary, Now string }
-type GoalsVars struct{ EngagementDescription, Now string }
+type PlannerVars struct{ Goal, Scope, AssetSummary, DataDir, Now string }
+type WorkerVars struct{ ProxyAddr, WorkerName, DataDir, Now string }
+type MainVars struct{ Goal, AssetSummary, FindingsSummary, DataDir, Now string }
+type GoalsVars struct{ EngagementDescription, DataDir, Now string }
 
 // nowStr is the server-local wall-clock string exposed as the universal {{.Now}}
 // prompt variable. renderSystem runs on every agent turn/round, so this is fresh