Fold NOTES.md into README.md

NOTES.md was written before the code, to hold the intent and the checked
facts about the environment. Everything in it that still applies now
lives in README.md, and the parts that do not -- the order of work, the
fields to verify against real hook input -- were answered by building it.

Two notes were kept rather than dropped: the rejection of desktop
notifications, which is a decision that would otherwise be re-litigated,
and the reasoning for hiding sub-agents. The original text stays in
history at 7fc7f63.

README.md

@@ -24,6 +24,11 @@ The panel shows the **highest-priority** session and, when others share that
 state, a `+N` count. Within a state the **oldest** one wins: the session you
 have forgotten about is the one that has been waiting longest, never the latest.
 
+There are deliberately **no desktop notifications**. `notify-send` from a hook
+would have been far cheaper to build, and it is the wrong shape: a session that
+has been waiting twenty minutes needs to stay visible, and a notification is
+gone the moment it is dismissed. The panel is the whole channel.
+
 ## Install
 
 ```sh