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.

mcp_serve.py

@@ -433,7 +433,7 @@ def create_mcp_server(event_bridge: Optional[EventBridge] = None) -> "FastMCP":
     if not _MCP_SERVER_AVAILABLE:
         raise ImportError(
             "MCP server requires the 'mcp' package. "
-            "Install with: pip install 'hermes-agent[mcp]'"
+            f"Install with: {sys.executable} -m pip install 'mcp'"
         )
 
     mcp = FastMCP(
@@ -838,7 +838,7 @@ def run_mcp_server(verbose: bool = False) -> None:
     if not _MCP_SERVER_AVAILABLE:
         print(
             "Error: MCP server requires the 'mcp' package.\n"
-            "Install with: pip install 'hermes-agent[mcp]'",
+            f"Install with: {sys.executable} -m pip install 'mcp'",
             file=sys.stderr,
         )
         sys.exit(1)