fix: prevent TUI gateway stdin EOF crash across all TUI-context subprocess calls

When Hermes runs in TUI mode, the gateway child process communicates with
the Node.js parent over a JSON-RPC protocol on stdin. Subprocess calls that
inherit this stdin fd can trigger a race condition where the child's stdin
read returns EOF, causing the gateway to exit cleanly (exit code 0) mid-tool-
execution.

This is the same root cause as issue #14036 (byterover plugin) and PR #39257
(SSH environment backend). This commit applies the fix — stdin=subprocess.DEVNULL
— to all 85 subprocess.run() and subprocess.Popen() calls that execute inside
the TUI gateway child process.

Scope: TUI-context code only (agent/, tools/, plugins/, tui_gateway/server.py).
CLI code (cli.py, hermes_cli/), tests, scripts, and gateway process management
are excluded — they don't run inside the TUI child and inherit the terminal's
stdin, not the JSON-RPC pipe.

85 call sites across 28 files. All files pass syntax check.

tools/tts_tool.py

@@ -693,6 +693,7 @@ def _terminate_command_tts_process_tree(proc: subprocess.Popen) -> None:
                 stdout=subprocess.DEVNULL,
                 stderr=subprocess.DEVNULL,
                 timeout=5,
+                stdin=subprocess.DEVNULL,
             )
         except Exception:
             proc.kill()
@@ -745,7 +746,7 @@ def _run_command_tts(command: str, timeout: float) -> subprocess.CompletedProces
     else:
         popen_kwargs["start_new_session"] = True
 
-    proc = subprocess.Popen(command, **popen_kwargs)
+    proc = subprocess.Popen(command, **popen_kwargs, stdin=subprocess.DEVNULL)
     try:
         stdout, stderr = proc.communicate(timeout=timeout)
     except subprocess.TimeoutExpired as exc:
@@ -882,6 +883,7 @@ def _convert_to_opus(mp3_path: str) -> Optional[str]:
             ["ffmpeg", "-i", mp3_path, "-acodec", "libopus",
              "-ac", "1", "-b:a", "64k", "-vbr", "off", ogg_path, "-y"],
             capture_output=True, timeout=30,
+            stdin=subprocess.DEVNULL,
         )
         if result.returncode != 0:
             logger.warning("ffmpeg conversion failed with return code %d: %s", 
@@ -1504,7 +1506,7 @@ def _generate_gemini_tts(text: str, output_path: str, tts_config: Dict[str, Any]
                 ]
             else:
                 cmd = [ffmpeg, "-i", wav_path, "-y", "-loglevel", "error", output_path]
-            result = subprocess.run(cmd, capture_output=True, timeout=30)
+            result = subprocess.run(cmd, capture_output=True, timeout=30, stdin=subprocess.DEVNULL)
             if result.returncode != 0:
                 stderr = result.stderr.decode("utf-8", errors="ignore")[:300]
                 raise RuntimeError(f"ffmpeg conversion failed: {stderr}")
@@ -1587,7 +1589,7 @@ def _generate_neutts(text: str, output_path: str, tts_config: Dict[str, Any]) ->
         "--device", device,
     ]
 
-    result = subprocess.run(cmd, capture_output=True, text=True, timeout=120)
+    result = subprocess.run(cmd, capture_output=True, text=True, timeout=120, stdin=subprocess.DEVNULL)
     if result.returncode != 0:
         stderr = result.stderr.strip()
         # Filter out the "OK:" line from stderr
@@ -1599,7 +1601,7 @@ def _generate_neutts(text: str, output_path: str, tts_config: Dict[str, Any]) ->
         ffmpeg = shutil.which("ffmpeg")
         if ffmpeg:
             conv_cmd = [ffmpeg, "-i", wav_path, "-y", "-loglevel", "error", output_path]
-            subprocess.run(conv_cmd, check=True, timeout=30)
+            subprocess.run(conv_cmd, check=True, timeout=30, stdin=subprocess.DEVNULL)
             os.remove(wav_path)
         else:
             # No ffmpeg — just rename the WAV to the expected path
@@ -1670,6 +1672,7 @@ def _resolve_piper_voice_path(voice: str, download_dir: Path) -> str:
             [_sys.executable, "-m", "piper.download_voices", voice,
              "--download-dir", str(download_dir)],
             capture_output=True, text=True, timeout=300,
+            stdin=subprocess.DEVNULL,
         )
     except subprocess.TimeoutExpired as exc:
         raise RuntimeError(
@@ -1757,7 +1760,7 @@ def _generate_piper_tts(text: str, output_path: str, tts_config: Dict[str, Any])
         ffmpeg = shutil.which("ffmpeg")
         if ffmpeg:
             conv_cmd = [ffmpeg, "-i", wav_path, "-y", "-loglevel", "error", output_path]
-            subprocess.run(conv_cmd, check=True, timeout=30)
+            subprocess.run(conv_cmd, check=True, timeout=30, stdin=subprocess.DEVNULL)
             try:
                 os.remove(wav_path)
             except OSError:
@@ -1823,7 +1826,7 @@ def _generate_kittentts(text: str, output_path: str, tts_config: Dict[str, Any])
         ffmpeg = shutil.which("ffmpeg")
         if ffmpeg:
             conv_cmd = [ffmpeg, "-i", wav_path, "-y", "-loglevel", "error", output_path]
-            subprocess.run(conv_cmd, check=True, timeout=30)
+            subprocess.run(conv_cmd, check=True, timeout=30, stdin=subprocess.DEVNULL)
             os.remove(wav_path)
         else:
             # No ffmpeg — rename the WAV to the expected path