Offer Stop for detached background shells in Agent Host chats #341329

In Agent Host Copilot chats, the Background Shells pill lists both attached and detached background shells. Since #340097, an attached shell can be stopped with Stop Shell in its details, but a detached one can't. Ending a detached dev server or watcher means finding and killing its process outside VS Code.

Offer Stop Shell for detached background shells too:

• Show Stop for a detached shell whenever the Copilot SDK can stop it: session.rpc.tasks.list reports the shell's process ID, and session.rpc.tasks.cancel stops it.
• Show a detached shell's details like an attached shell's: the command and its status in the same card, with Stop beside the command, even though there's no live output to show.
• Make Stop look the same in the editor window and the Agents window.

Stopping a detached shell shouldn't affect the chat's turn, since detached shells don't hold the turn open.