feat(agents): dock a session in the Agents tool window (#588)

Agent sessions can be opened in the Agents tool window beside the editor (Open in Tool Window / Open in Editor in the task menus, or by dragging between the editor and the tool window); opening in an editor tab stays the default.

Fixes in review:
- A docked conversation kept the previous task's subagent view after switching tasks: the session's local state is now keyed by task.
- Moving a conversation between the editor and the tool window lost its scroll position: position and follow-output are kept per conversation (task or subagent).
- In a short bottom tool window the message area shrank to a few pixels: wide windows place the task list beside the conversation, narrow ones cap the list height.
- Drag and drop and list selection now go through the agents.dockTask / undockTask / openTask actions.

Co-authored-by: a-rookie-of-C-language <[email protected]>

src/platform/locales/text/zh.json

@@ -1359,10 +1359,12 @@
   "Open console for tool output": "为工具输出打开控制台",
   "Open documents": "打开的文档",
   "Open in Browser": "在浏览器中打开",
+  "Open in Editor": "在编辑器中打开",
   "Open in Find Tool Window": "在“查找”工具窗口中打开",
   "Open in Find Window": "在查找窗口中打开",
   "Open in Opposite Group": "在相对组中打开",
   "Open in New Window": "在新窗口中打开",
+  "Open in Tool Window": "在工具窗口中打开",
   "Open in Terminal": "在终端中打开",
   "Open in editor": "在编辑器中打开",
   "Open moved files in editor": "在编辑器中打开已移动的文件",
@@ -4816,6 +4818,8 @@
   "Add to .git/info/exclude": "添加到 .git/info/exclude",
   "Delete Agent Tasks": "删除 Agent 任务",
   "Open Agent Task": "打开 Agent 任务",
+  "Open Agent Task in Editor": "在编辑器中打开 Agent 任务",
+  "Open Agent Task in Tool Window": "在工具窗口中打开 Agent 任务",
   "Archive Agent Tasks": "归档 Agent 任务",
   "Delete {count} agent tasks? Their branches and files outside worktrees are kept.": "是否删除这 {count} 个 Agent 任务？它们的分支及 worktree 以外的文件将保留。",
   "Also remove their worktree folders": "同时删除它们的 worktree 目录",