Add a QMP client module for the web UI

Wraps the connect/negotiate/command dance with a lock, since only one client
can hold the QMP socket at a time. Events interleave with replies, so command()
reads until it sees the matching return rather than trusting the first message.

Connection failure carries the reason and the likely cause: key.py already
holding the socket.

.gitignore

@@ -10,3 +10,6 @@ build/
 
 # Screenshots used in the README are committed on purpose.
 !docs/screenshots/*.png
+
+# Plan documents from the plan skill: local working notes.
+.hermes/