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.

gateway/run.py

@@ -5520,8 +5520,7 @@ class GatewayRunner:
             if "pynacl" in err_lower or "nacl" in err_lower or "davey" in err_lower:
                 return (
                     "Voice dependencies are missing (PyNaCl / davey). "
-                    "Install or reinstall Hermes with the messaging extra, e.g. "
-                    "`pip install hermes-agent[messaging]`."
+                    f"Install with: `{sys.executable} -m pip install PyNaCl`"
                 )
             return f"Failed to join voice channel: {e}"