feat(agents): show the Changes pane only when it's useful, toggled from the task header (#421)

The Changes pane took a fixed column next to every agent conversation, including tasks that changed nothing. It is now shown while the task has changes (so it appears when the agent first edits a file) and toggled from a Changes button with the file count in the task header; once toggled, the choice is kept per task.

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

src/platform/locales/text/zh.json

@@ -906,6 +906,7 @@
   "Hide": "隐藏",
   "Hide Active Tool Window": "隐藏活动工具窗口",
   "Hide All Tool Windows": "隐藏所有工具窗口",
+  "Hide changes": "隐藏更改",
   "Hide Coverage": "隐藏覆盖率",
   "Hide Library Frames": "隐藏库帧",
   "Hide Replace": "隐藏替换",
@@ -1916,6 +1917,7 @@
   "Show Commit in Git Log": "在 Git 日志中显示提交",
   "Show Container Log": "显示容器日志",
   "Show Context Actions": "显示上下文操作",
+  "Show changes": "显示更改",
   "Show Coverage": "显示覆盖率",
   "Show Debug Tool Window": "显示调试工具窗口",
   "Show Diff": "显示差异",