Add helpful ImportError messages for optional dependencies

When optional dependencies are missing, raise ImportError with
installation
instructions pointing to the relevant extras group (e.g. `[messaging]`,
`[cli]`, `[mcp]`, etc.) instead of letting the import fail silently.

tools/mcp_tool.py

@@ -1501,7 +1501,13 @@ def _snapshot_child_pids() -> set:
         pass
 
     # Fallback: psutil
-    import psutil
+    try:
+        import psutil
+    except ImportError:
+        raise ImportError(
+            "psutil is required for MCP child process tracking. "
+            "Install with: pip install hermes-agent[mcp]"
+        ) from None
     try:
         return {c.pid for c in psutil.Process(my_pid).children()}
     except psutil.Error: