fix: point optional-dep install hints at the venv's python (#11938)

Error messages that tell users to install optional extras now use
{sys.executable} -m pip install ... instead of a bare 'pip install
hermes-agent[extra]' string.  Under the curl installer, bare 'pip'
resolves to system pip, which either fails with PEP 668
externally-managed-environment or installs into the wrong Python.

Affects: hermes dashboard, hermes web server startup, mcp_serve,
hermes doctor Bedrock check, CLI voice mode, voice_mode tool runtime
error, Discord voice-channel join failure message.

tools/voice_mode.py

@@ -15,6 +15,7 @@ import platform
 import re
 import shutil
 import subprocess
+import sys
 import tempfile
 import threading
 import time
@@ -582,8 +583,7 @@ class AudioRecorder:
         except (ImportError, OSError) as e:
             raise RuntimeError(
                 "Voice mode requires sounddevice and numpy.\n"
-                "Install with: pip install sounddevice numpy\n"
-                "Or: pip install hermes-agent[voice]"
+                f"Install with: {sys.executable} -m pip install sounddevice numpy"
             ) from e
 
         with self._lock: