Archived session's background process keeps running after archive

Description

Archiving a session (via the session archive action / archive_session) is documented to stop the session's underlying process ("Archiving stops the session's process and (by default) cleans up its worktree"). In practice, after archiving a session in the UI, the session's process keeps running in the background — it no longer appears as active in the UI, but it continues consuming resources (and presumably tokens) as if still live.

Expected behavior

Archiving a session should terminate its running process, per the documented behavior.

Actual behavior

The session is marked archived / disappears from the active list, but its process is not killed and keeps executing.

Impact

• Wasted compute/token usage from "zombie" sessions believed to be stopped
• Confusing state: UI says archived, but the session is still doing work