SIG_DFL for SIGPIPE means the kernel reaps the gateway subprocess the
instant a background thread (TTS playback, silence callback, voice
status emitter) writes to a stdout the TUI stopped reading — before
the Python interpreter can run excepthook, threading.excepthook,
atexit, or the entry.py post-loop _log_exit.
Replace the three SIG_DFL / SIG_IGN bindings with a _log_signal
handler that:
- records which signal (SIGPIPE / SIGTERM / SIGHUP) fired and when;
- dumps the main-thread stack at signal delivery AND every live
thread's stack via sys._current_frames — the background-thread
write that provoked SIGPIPE is almost always visible here;
- writes everything to ~/.hermes/logs/tui_gateway_crash.log and prints
a [gateway-signal] breadcrumb to stderr so the TUI Activity surfaces
it as well.
SIGINT stays ignored (TUI handles Ctrl+C for the user).