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/features/agents/Session.module.css

@@ -16,6 +16,13 @@
   border-right: 1px solid var(--border);
 }
 
+/* The conversation hugs its content; spare space stays below the composer
+   instead of inside the chat. */
+.streamArea,
+.stream {
+  flex: 0 1 auto;
+}
+
 .header {
   display: flex;
   align-items: center;