/stop was calling suspend_session() which marked the session for auto-reset
on the next message. This meant users lost their conversation history every
time they stopped a running agent — especially painful for untitled sessions
that can't be resumed by name.
Now /stop just interrupts the agent and cleans the session lock. The session
stays intact so users can continue the conversation.
The suspend behavior was introduced in #7536 to break stuck session resume
loops on gateway restart. That case is already handled by
suspend_recently_active() which runs at gateway startup, so removing it from
/stop doesn't regress the original fix.
952a885fbf