Report the running build's version to the master

The version is the checkout's HEAD commit (short hash + commit time),
read once at startup — updates arrive as `git pull`, so there is no
release number to bump. It rides on the socket hello, socket reports,
and the HTTP report fallback; the field is optional in the protocol so
older masters ignore it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -101,6 +101,13 @@ instance does two things with them: it reports its firm and account counts every
 30 seconds, and it holds a WebSocket open to `<url>/ws/instance` so the master
 can drive it.
 
+Every report also carries this checkout's version — the HEAD commit and its
+time, since updates arrive as `git pull` and there is no release number to
+bump. The master shows it in a **Version** column and flags any machine running
+something older than the newest build in the fleet, which is how you check at a
+glance that every machine took the last update. An instance showing *old build*
+predates version reporting entirely.
+
 What the master can trigger here:
 
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