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).

cli.py

@@ -1017,9 +1017,7 @@ _IMAGE_EXTENSIONS = frozenset({
 })
 
 
-def _is_termux_environment() -> 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_environment
 
 
 def _termux_example_image_path(filename: str = "cat.png") -> str:

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):

hermes_constants.py

@@ -93,6 +93,16 @@ def parse_reasoning_effort(effort: str) -> dict | None:
     return None
 
 
+def is_termux() -> bool:
+    """Return True when running inside a Termux (Android) environment.
+
+    Checks ``TERMUX_VERSION`` (set by Termux) or the Termux-specific
+    ``PREFIX`` path.  Import-safe — no heavy deps.
+    """
+    prefix = os.getenv("PREFIX", "")
+    return bool(os.getenv("TERMUX_VERSION") or "com.termux/files/usr" in prefix)
+
+
 OPENROUTER_BASE_URL = "https://openrouter.ai/api/v1"
 OPENROUTER_MODELS_URL = f"{OPENROUTER_BASE_URL}/models"
 OPENROUTER_CHAT_URL = f"{OPENROUTER_BASE_URL}/chat/completions"

tools/browser_tool.py

@@ -285,9 +285,7 @@ def _get_cloud_provider() -> Optional[CloudBrowserProvider]:
     return _cached_cloud_provider
 
 
-def _is_termux_environment() -> 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_environment
 
 
 def _browser_install_hint() -> str:

tools/voice_mode.py

@@ -48,9 +48,7 @@ def _audio_available() -> bool:
         return False
 
 
-def _is_termux_environment() -> 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_environment
 
 
 def _voice_capture_install_hint() -> str: