chore(tui): capture signal-triggered gateway exits in crash log

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).

tui_gateway/entry.py

@@ -3,10 +3,50 @@ import os
 import signal
 import sys
 import time
+import traceback
 
 from tui_gateway.server import _CRASH_LOG, dispatch, resolve_skin, write_json
 
-signal.signal(signal.SIGPIPE, signal.SIG_DFL)
+
+def _log_signal(signum: int, frame) -> None:
+    """Capture WHICH thread and WHERE a termination signal hit us.
+
+    SIG_DFL for SIGPIPE kills the process silently the instant any
+    background thread (TTS playback, beep, voice status emitter, etc.)
+    writes to a stdout the TUI has stopped reading.  Without this
+    handler the gateway-exited banner in the TUI has no trace — the
+    crash log never sees a Python exception because the kernel reaps
+    the process before the interpreter runs anything.
+    """
+    name = {
+        signal.SIGPIPE: "SIGPIPE",
+        signal.SIGTERM: "SIGTERM",
+        signal.SIGHUP: "SIGHUP",
+    }.get(signum, f"signal {signum}")
+    try:
+        os.makedirs(os.path.dirname(_CRASH_LOG), exist_ok=True)
+        with open(_CRASH_LOG, "a", encoding="utf-8") as f:
+            f.write(
+                f"\n=== {name} received · {time.strftime('%Y-%m-%d %H:%M:%S')} ===\n"
+            )
+            if frame is not None:
+                f.write("main-thread stack at signal delivery:\n")
+                traceback.print_stack(frame, file=f)
+            # All live threads — signal may have been triggered by a
+            # background thread (write to broken stdout from TTS, etc.).
+            import threading as _threading
+            for tid, th in _threading._active.items():
+                f.write(f"\n--- thread {th.name} (id={tid}) ---\n")
+                f.write("".join(traceback.format_stack(sys._current_frames().get(tid))))
+    except Exception:
+        pass
+    print(f"[gateway-signal] {name}", file=sys.stderr, flush=True)
+    sys.exit(0)
+
+
+signal.signal(signal.SIGPIPE, _log_signal)
+signal.signal(signal.SIGTERM, _log_signal)
+signal.signal(signal.SIGHUP, _log_signal)
 signal.signal(signal.SIGINT, signal.SIG_IGN)