chore(tui): record gateway exit reason in crash log

Gateway exits weren't reaching the panic hook because entry.py calls
sys.exit(0) on broken stdout — clean termination, no exception.  That
left "gateway exited" in the TUI with zero forensic trail when pipe
breaks happened mid-turn.

Entry.py now tags each exit path — startup-write failure, parse-error-
response write failure, per-method response write failure, stdin EOF —
with a one-line entry in ~/.hermes/logs/tui_gateway_crash.log and a
gateway.stderr breadcrumb.  Includes the JSON-RPC method name on the
dispatch path, which is the only way to tell "died right after handling
voice.toggle on" from "died emitting the second message.complete".

tui_gateway/entry.py

@@ -1,19 +1,44 @@
 import json
+import os
 import signal
 import sys
+import time
 
-from tui_gateway.server import dispatch, resolve_skin, write_json
+from tui_gateway.server import _CRASH_LOG, dispatch, resolve_skin, write_json
 
 signal.signal(signal.SIGPIPE, signal.SIG_DFL)
 signal.signal(signal.SIGINT, signal.SIG_IGN)
 
 
+def _log_exit(reason: str) -> None:
+    """Record why the gateway subprocess is shutting down.
+
+    Three exit paths (startup write fail, parse-error-response write fail,
+    dispatch-response write fail, stdin EOF) all collapse into a silent
+    sys.exit(0) here.  Without this trail the TUI shows "gateway exited"
+    with no actionable clue about WHICH broken pipe or WHICH message
+    triggered it — the main reason voice-mode turns look like phantom
+    crashes when the real story is "TUI read pipe closed on this event".
+    """
+    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=== gateway exit · {time.strftime('%Y-%m-%d %H:%M:%S')} "
+                f"· reason={reason} ===\n"
+            )
+    except Exception:
+        pass
+    print(f"[gateway-exit] {reason}", file=sys.stderr, flush=True)
+
+
 def main():
     if not write_json({
         "jsonrpc": "2.0",
         "method": "event",
         "params": {"type": "gateway.ready", "payload": {"skin": resolve_skin()}},
     }):
+        _log_exit("startup write failed (broken stdout pipe before first event)")
         sys.exit(0)
 
     for raw in sys.stdin:
@@ -25,14 +50,19 @@ def main():
             req = json.loads(line)
         except json.JSONDecodeError:
             if not write_json({"jsonrpc": "2.0", "error": {"code": -32700, "message": "parse error"}, "id": None}):
+                _log_exit("parse-error-response write failed (broken stdout pipe)")
                 sys.exit(0)
             continue
 
+        method = req.get("method") if isinstance(req, dict) else None
         resp = dispatch(req)
         if resp is not None:
             if not write_json(resp):
+                _log_exit(f"response write failed for method={method!r} (broken stdout pipe)")
                 sys.exit(0)
 
+    _log_exit("stdin EOF (TUI closed the command pipe)")
+
 
 if __name__ == "__main__":
     main()