Recognise an empty tmux server's answer for a missing session

A running but empty server answers "no current target". hasSession only knew
"can't find session", "no server running" and "no sessions", so it returned
that as a real error. Kill then failed for a pane that was already gone, the
quarantine never cleared, and the worker's only session slot stayed pinned.

This is a side effect of dea56e4. Before the runtime became its own unit the
server exited with its last session and answered "no server running", which
hasSession already handled, so this reply had never been produced. AgentStatus
shares the same helper and now reports "exited" instead of erroring.

Found live: the F28 disposable task's session survived its own block on
93338b7, quarantined and unreapable.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011xsXyr5J1RACo71YeKG3Pu

internal/herdr/tmux_test.go

@@ -421,3 +421,27 @@ func TestPaneProgressIgnoresInputLines(t *testing.T) {
 		t.Fatal("real harness output left the progress digest unchanged")
 	}
 }
+
+// TestMissingSessionIsRecognisedOnAnEmptyRunningServer guards F32. A running
+// but empty tmux server answers "no current target", not "no server running".
+// The runtime only stays alive past its last pane since it became its own unit
+// running `tmux -D`, so this reply had never been seen before. Treating it as
+// a real error made Kill fail for a pane that was already gone, which left the
+// quarantine set and pinned the worker's only session slot.
+func TestMissingSessionIsRecognisedOnAnEmptyRunningServer(t *testing.T) {
+	for _, message := range []string{"no current target", "can't find session: x", "no server running"} {
+		bin := filepath.Join(t.TempDir(), "tmux")
+		script := "#!/bin/sh\necho \"" + message + "\" >&2\nexit 1\n"
+		if err := os.WriteFile(bin, []byte(script), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		b := &TmuxBackend{Binary: bin}
+		if err := b.Kill(context.Background(), Session{PaneID: "s:1.0"}); err != nil {
+			t.Fatalf("%q: killing an already-gone session failed: %v", message, err)
+		}
+		status, err := b.AgentStatus(context.Background(), Session{PaneID: "s:1.0"})
+		if err != nil || status != "exited" {
+			t.Fatalf("%q: status=%q err=%v, want exited", message, status, err)
+		}
+	}
+}