add exec, run and scale, and the agent skill

The skill documents how to use myos: what to check before touching a host
stack, how a reference resolves, what the three kinds of stack mean, and where
each failure comes from. Every command it shows was run against the fixtures
before being written down, which is how exec, run and scale turned out to be
missing.

AGENTS.md covers the other side: how to change myos without breaking the
deployments that already run it.

spec/golden/expected.cli/cmd-exec-service.txt

@@ -0,0 +1,2 @@
+docker compose -f @WD@/stack/host/fabio.yml -f @MYOS@/share/compose/networks.yml -p testhost --project-directory @WD@/stack/host exec fabio sh
+[exit 0]