feat(sidebar): replicate WorkBuddy task context menu (TODO 01)

Replace the sidebar delete button with the original task menu: pin/unpin,
rename, open in a separate window, open folder, archive and delete.

Source: REA ev_f5e9d71a676c453daab40928acfb33aeabae68ba57ffda342380cead7f1a1fe2
(WorkBuddy 5.8.3 task menu, open state) supplies the container/item geometry,
the two dividers, danger colour and every item icon.

Verified by scripts/verify-task-menu.cjs: 33 assertions in a real isolated
Electron app (real preload/IPC/filesystem), including menu geometry, anchor
offset, pin/rename/archive persistence, open-folder target and the separate
window query. typecheck, test:conversation and build pass.

src/shared/types.ts

@@ -138,6 +138,10 @@ export interface SessionMeta {
   expertId?: string | null
   agentMode?: import('./conversation').AgentMode
   thinkingLevel?: import('./workspaces').ThinkingLevel
+  /** 置顶时间（毫秒）。有值即视为已置顶，用于侧栏任务排序，不改变 createdAt/updatedAt 语义。 */
+  pinnedAt?: number
+  /** 归档时间（毫秒）。归档任务从默认任务列表移出，但不删除消息与工作目录。 */
+  archivedAt?: number
 }
 
 /** 工作目录信息 */