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/main.py

@@ -6029,7 +6029,7 @@ def cmd_dashboard(args):
         import uvicorn  # noqa: F401
     except ImportError:
         print("Web UI dependencies not installed.")
-        print("Install them with:  pip install hermes-agent[web]")
+        print(f"Install them with:  {sys.executable} -m pip install 'fastapi' 'uvicorn[standard]'")
         sys.exit(1)
 
     if not _build_web_ui(PROJECT_ROOT / "web", fatal=True):