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/app-scale.txt

@@ -1,26 +1,4 @@
-ERROR: unknown command: scale
-Usage: myos [options] <command> [stack...] [VAR=value...] [-- args...]
-
-Options:
- -C DIR work in DIR instead of the current directory
- -e ENV environment (default: local, or ENV from the config)
- -H HOSTS run on remote hosts instead (comma separated, or "all")
- -n, --dry-run print the commands instead of running them
- -v, --verbose show what myos does
- -d, --debug show every command
- -h, --help this help
-
-Commands:
- up down start stop restart recreate manage the containers of a stack
- ps logs config exec run inspect and enter them
- ls [--groups] list the stacks myos can see
- env [VAR...] show resolved variables
- doctor check the installation
- version print the myos version
-
-Stacks:
- myos up the stack of the current directory
- myos up host a group, expanded from host=... in a .env or .mk
- myos up host/fabio a single stack
- myos up postgres:9.6 a versioned stack
-[exit 2]
+docker network create tester-local
+docker network create testhost
+docker compose -f @WD@/docker-compose.yml -f @WD@/docker-compose.local.yml -f @WD@/docker/docker-compose.yml -f @MYOS@/share/compose/networks.yml -p tester-wd-local --project-directory @WD@ up -d --scale app=2
+[exit 0]