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/chat.go

@@ -57,14 +57,15 @@ func chatWorkDirSpec(workDir string) string {
 
 // chatSystem renders the DB-managed prompt body for agentKey. Custom agents have
 // no per-key in-code default, so DefaultAssistantPrompt is the render fallback.
-func chatSystem(agentKey, workDir string) string {
-	return renderSystem(agentKey, DefaultAssistantPrompt, chatVars{Now: nowStr()}) + chatWorkDirSpec(workDir)
+func chatSystem(agentKey, dataDir, workDir string) string {
+	return renderSystem(agentKey, DefaultAssistantPrompt, chatVars{DataDir: dataDir, Now: nowStr()}) + chatWorkDirSpec(workDir)
 }
 
 // chatVars carries the runtime variables a custom agent's prompt may reference.
-// Now (server wall-clock, refreshed each turn) is the only one today; any other
-// {{.X}} fails to render and falls back to DefaultAssistantPrompt.
-type chatVars struct{ Now string }
+// DataDir (server data root) + Now (server wall-clock, refreshed each turn) are the
+// universal ones; any other {{.X}} fails to render and falls back to
+// DefaultAssistantPrompt.
+type chatVars struct{ DataDir, Now string }
 
 // Chat runs ONE turn of a conversation with the agent identified by agentKey,
 // resuming prior history keyed by sessionID. maxTurns is the per-turn agent step
@@ -94,7 +95,7 @@ func (c *ChatAgent) Chat(ctx context.Context, agentKey, sessionID, message strin
 	tools, def, cleanup := AugmentTools(ctx, agentKey, base)
 	defer cleanup()
 
-	system, boundary := deferredSystem(chatSystem(agentKey, sessionWorkDir), def)
+	system, boundary := deferredSystem(chatSystem(agentKey, c.workDir, sessionWorkDir), def)
 	opts := agentcore.Options{
 		Provider:        c.prov,
 		SystemPrompt:    system,