fix: deduplicate _is_termux() into hermes_constants.is_termux()

Replace 6 identical copies of the Termux detection function across
cli.py, browser_tool.py, voice_mode.py, status.py, doctor.py, and
gateway.py with a single shared implementation in hermes_constants.py.

Each call site imports with its original local name to preserve all
existing callers (internal references and test monkeypatches).

hermes_cli/doctor.py

@@ -54,9 +54,7 @@ _PROVIDER_ENV_HINTS = (
 )
 
 
-def _is_termux() -> bool:
-    prefix = os.getenv("PREFIX", "")
-    return bool(os.getenv("TERMUX_VERSION") or "com.termux/files/usr" in prefix)
+from hermes_constants import is_termux as _is_termux
 
 
 def _python_install_cmd() -> str:

hermes_cli/gateway.py

@@ -226,9 +226,7 @@ def is_linux() -> bool:
     return sys.platform.startswith('linux')
 
 
-def is_termux() -> bool:
-    prefix = os.getenv("PREFIX", "")
-    return bool(os.getenv("TERMUX_VERSION") or "com.termux/files/usr" in prefix)
+from hermes_constants import is_termux
 
 
 def supports_systemd_services() -> bool:

hermes_cli/status.py

@@ -79,9 +79,7 @@ def _effective_provider_label() -> str:
     return provider_label(effective)
 
 
-def _is_termux() -> bool:
-    prefix = os.getenv("PREFIX", "")
-    return bool(os.getenv("TERMUX_VERSION") or "com.termux/files/usr" in prefix)
+from hermes_constants import is_termux as _is_termux
 
 
 def show_status(args):