Add pinned and archived flags to Conversation

Why: the approved task-centric sidebar (proposal 2026-09-30, option A) needs
server-authoritative pin and archive state on conversations; client-side
storage would be fake state against the event-sourced model.

Evidence: go test ./... passes; omitempty keeps wire format and golden
samples byte-identical for zero values (backward compatible).

Impact: additive optional fields; consumers ignoring unknown fields are
unaffected (forward-compat principle). Backend consumes via submodule bump.

Refs: Agent-System-Documents docs/proposals/2026-09-30-ui-redesign-and-auth.md (P6/P8, R1)

model.go

@@ -31,6 +31,8 @@ type Conversation struct {
 	Owner     ID           `json:"owner,omitempty"` // 冗余自项目，查询过滤用
 	Title     string       `json:"title"`
 	ProfileID ID           `json:"profileId,omitempty"` // 空为默认画像
+	Pinned    bool         `json:"pinned,omitempty"`    // 侧栏置顶（方案 A P6/P8：本端组织偏好，服务端权威）
+	Archived  bool         `json:"archived,omitempty"`  // 归档：列表默认隐藏，数据保留
 	CreatedAt time.Time    `json:"createdAt"`
 	UpdatedAt time.Time    `json:"updatedAt"`
 }

ts/types.ts

@@ -13,6 +13,7 @@ export interface Project {
 
 export interface Conversation {
   id: ID; projectId: ID; owner?: ID; title: string; profileId?: ID;
+  pinned?: boolean; archived?: boolean;
   createdAt: string; updatedAt: string;
 }