Escalate agent Close to SIGKILL so it terminates in one action

Agent 'Close' (agent-close) sent a single SIGTERM via Session.Kill and
never escalated, so an agent that traps/ignores SIGTERM (e.g. opencode)
stayed in the running tab bar until the user closed it again. Add
Session.Terminate, which reuses terminateAndWait (SIGTERM, wait, then
SIGKILL) but preserves the session entry so the exited pane stays
readable, and route handleChildClose's agent path through it in a
goroutine to keep the UI responsive during the stop timeout.

Resolves the opencode double-close TODO item.

internal/app/session.go

@@ -395,6 +395,20 @@ func (s *Session) Close(id string, sig syscall.Signal) error {
 	return nil
 }
 
+// Terminate stops a live child with SIGTERM/SIGKILL escalation but
+// leaves its session entry intact so callers can keep showing the
+// exited pane.
+func (s *Session) Terminate(id string, sig syscall.Signal) error {
+	c := s.FindChild(id)
+	if c == nil {
+		return fmt.Errorf("no such process %q", id)
+	}
+	if c.IsLive() {
+		terminateAndWait(c, sig, childStopTimeout)
+	}
+	return nil
+}
+
 // mintUniqueIDLocked mints an opaque process_id (SPEC §7) and retries
 // if it collides with an existing entry. Caller holds s.mu.
 func (s *Session) mintUniqueIDLocked() string {