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.

hermes_cli/web_server.py

@@ -56,7 +56,7 @@ try:
 except ImportError:
     raise SystemExit(
         "Web UI requires fastapi and uvicorn.\n"
-        "Run 'hermes web' to auto-install, or: pip install hermes-agent[web]"
+        f"Install with: {sys.executable} -m pip install 'fastapi' 'uvicorn[standard]'"
     )
 
 WEB_DIST = Path(__file__).parent / "web_dist"