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/app.go

@@ -2345,11 +2345,9 @@ func (st *uiState) handleChildRename(childID, newName string) {
 	st.drawStatusLine()
 }
 
-// handleChildClose removes a child entry entirely. For agents this is
-// equivalent to a SIGTERM kill (the entry is ephemeral and disappears
-// from the session once the PTY exits). For command processes it's
-// equivalent to the MCP close_process tool: SIGKILL if alive, then
-// drop the entry so it stops appearing in the switch/restart lists.
+// handleChildClose removes a child entry entirely for process deletes.
+// For agent Close, it terminates the PTY with escalation but preserves
+// the exited pane so the user can still read the corpse.
 func (st *uiState) handleChildClose(childID string, kill bool) {
 	if childID == "" {
 		st.repaintFocused()
@@ -2364,7 +2362,11 @@ func (st *uiState) handleChildClose(childID string, kill bool) {
 	if kill {
 		_ = st.sess.Close(childID, syscall.SIGKILL)
 	} else {
-		_ = st.sess.Kill(childID, syscall.SIGTERM)
+		go func() {
+			if err := st.sess.Terminate(childID, syscall.SIGTERM); err != nil {
+				logf("terminate child %s: %v", childID, err)
+			}
+		}()
 	}
 	st.repaintFocused()
 	st.drawTabBar()

internal/app/palette.go

@@ -267,7 +267,7 @@ func (p *paletteState) buildItems(macro string) []paletteItem {
 				out = append(out,
 					paletteItem{label: "Rename", hint: "rename agent · " + name,
 						action: paletteAction{kind: "agent-rename-form", childID: c.ID}, group: groupFocused},
-					paletteItem{label: "Close", hint: "close agent · " + name + " (SIGTERM)",
+					paletteItem{label: "Close", hint: "close agent · " + name + " (SIGTERM, escalates)",
 						action: paletteAction{kind: "agent-close", childID: c.ID}, group: groupFocused},
 				)
 			case KindTerminal:

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 {

internal/app/session_test.go

@@ -1,6 +1,7 @@
 package app
 
 import (
+	"strings"
 	"syscall"
 	"testing"
 	"time"
@@ -101,6 +102,50 @@ func TestSpawnInstallsIdleDetectionBeforePublish(t *testing.T) {
 	}
 }
 
+func TestTerminateEscalatesWithoutRemovingEntry(t *testing.T) {
+	sess := NewSession(t.TempDir(), "test")
+	c, err := sess.Spawn(SpawnSpec{
+		Kind: KindAgent,
+		Argv: []string{"sh", "-c", "trap '' TERM; echo ready; while :; do sleep 1; done"},
+	}, 80, 24)
+	if err != nil {
+		t.Fatalf("spawn: %v", err)
+	}
+	t.Cleanup(func() {
+		if c.IsLive() {
+			_ = c.signal(syscall.SIGKILL)
+		}
+	})
+	waitUntilLive(t, c)
+	waitForStreamText(t, c, "ready")
+
+	start := time.Now()
+	if err := sess.Terminate(c.ID, syscall.SIGTERM); err != nil {
+		t.Fatalf("Terminate: %v", err)
+	}
+	if elapsed := time.Since(start); elapsed < childStopTimeout {
+		t.Fatalf("Terminate returned before SIGKILL fallback: elapsed=%s timeout=%s", elapsed, childStopTimeout)
+	}
+	waitUntilNotLive(t, c)
+
+	if got := sess.FindChild(c.ID); got == nil {
+		t.Fatalf("Terminate removed child entry %s", c.ID)
+	}
+}
+
+func waitForStreamText(t *testing.T, c *Child, want string) {
+	t.Helper()
+	deadline := time.Now().Add(5 * time.Second)
+	for time.Now().Before(deadline) {
+		b, _ := c.StreamRead(0)
+		if strings.Contains(string(b), want) {
+			return
+		}
+		time.Sleep(20 * time.Millisecond)
+	}
+	t.Fatalf("child %s never wrote %q", c.ID, want)
+}
+
 func waitUntilLive(t *testing.T, c *Child) {
 	t.Helper()
 	deadline := time.Now().Add(5 * time.Second)