Summary

Fixes #32377 by tightening dashboard PTY teardown so tui_gateway.slash_worker descendants do not survive repeated /api/pty disconnects.

Root Cause

The dashboard PTY close path signalled only the PTY root process. If that process exited while descendants such as the TUI gateway's slash worker were still alive, the worker could become orphaned and accumulate under the dashboard service cgroup. Separately, _SlashWorker.close() did not wait after SIGKILL, and the worker process had no Linux parent-death guard for abrupt gateway exits.

Changes

• Track the PTY child process group in PtyBridge and send SIGHUP -> SIGTERM -> SIGKILL to that group during close.
• Add Linux PR_SET_PDEATHSIG setup in tui_gateway.slash_worker itself, avoiding preexec_fn from the multithreaded gateway parent.
• Make _SlashWorker.close() wait after SIGKILL and close stdio pipes in all exit paths.
• Add regression tests for PTY grandchild cleanup, slash worker parent-death signal installation, and SIGKILL wait/reap behavior.

Validation

• pytest tests/hermes_cli/test_pty_bridge.py::TestPtyBridgeClose::test_close_terminates_child_process_group -q
• pytest tests/tui_gateway/test_protocol.py::test_slash_worker_child_installs_linux_parent_death_signal tests/tui_gateway/test_protocol.py::test_slash_worker_close_waits_after_kill -q
• pytest tests/hermes_cli/test_pty_bridge.py tests/hermes_cli/test_web_server.py::TestPtyWebSocket -q
• pytest tests/tui_gateway/test_protocol.py -q
• pytest tests/test_tui_gateway_server.py::test_session_create_close_race_does_not_orphan_worker tests/test_tui_gateway_server.py::test_session_create_no_race_keeps_worker_alive -q
• python -m py_compile hermes_cli/pty_bridge.py tui_gateway/server.py tui_gateway/slash_worker.py
• git diff --check

Note: pytest tests/test_tui_gateway_server.py::test_browser_manage_connect_default_local_reports_launch_hint -q fails in this checkout independently of these changes; the assertion expects a browser-launch hint that the current local environment/code path does not emit.