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.

skills/myos/SKILL.md

@@ -0,0 +1,91 @@
+---
+name: myos
+description: Deploy, inspect and troubleshoot docker compose stacks on a server or a fleet with the myos CLI (Make Your Own Stack). Use when asked to install myos, to start/stop/debug a stack (a host singleton, an application, a user stack), to add a stack to the catalogue, or to work on the myos framework itself.
+---
+
+# myos
+
+myos runs docker compose stacks. It decides which compose files to load, under
+which project name, with which variables. Everything else is docker.
+
+## Is this the right tool
+
+Use it when the repository or the host has one of:
+- a `stack/` directory holding compose files,
+- a `Makefile` that includes `make/include.mk`,
+- `/etc/conf.d/myos` or `/etc/default/myos`,
+- the `myos` command on PATH.
+
+Otherwise call `docker compose` directly.
+
+## Look before you touch
+
+```sh
+myos doctor              # requirements, config layers, stack path
+myos ls                  # stacks reachable from here
+myos ls --groups         # groups and what they expand to
+myos env                 # resolved variables
+myos -n up host          # print what `up` would run, without running it
+```
+
+**Always run `myos -n` before `up`, `down` or `restart` on a `host/` stack.**
+Host stacks bind privileged ports and are shared by everything on the machine:
+taking `host/fabio` down drops every site the machine serves.
+
+## Working loop
+
+```sh
+myos ls                       # find the stack
+myos -n config <stack>        # check the file list and the project name
+myos up <stack>               # create and start
+myos ps <stack>               # what is running
+myos logs <stack>             # follow the logs
+myos exec <stack> -- sh       # a shell in the service named after the stack
+myos down <stack>             # remove the containers
+```
+
+## Naming a stack
+
+A reference is `[<group>/]<name>[:<version>]`, or a path.
+
+```sh
+myos up                    # the current directory, when it holds a compose file
+myos up host               # a group: expands to host/consul host/fabio ...
+myos up host/fabio         # one stack
+myos up postgres:9.6       # the 9.6 overlay of the postgres stack
+myos -C /opt/app up        # somewhere else
+myos -e master up          # in another environment
+```
+
+Stacks are looked up along the stack path, project first:
+`./stack`, `../stack`, `~/.local/share/myos/stack`, `/usr/local/share/myos/stack`,
+`/usr/share/myos/stack`. `myos doctor` prints the resolved path.
+
+## Three kinds of stack
+
+| kind | reference | compose project | meaning |
+|---|---|---|---|
+| host | `host/*` | the hostname | one per machine: binds 80/443, consul, certbot. Shared. Treat as production. |
+| user | `User/*` | the user identity | one per user: ssh-agent, personal services |
+| app | anything else | `<user>-<env>-<app>` | many per machine, one per user and environment |
+
+Set `MYOS_PROJECT_FORMAT=user-app-env` on a deployment created before the
+rename, otherwise its containers and volumes look like a different project.
+See `references/conventions.md`.
+
+## Rules
+
+- Never run `myos clean` on a host stack: it removes images **and volumes**,
+  including the certificates.
+- Secrets belong in a file outside the repository, never in a compose file.
+- A stack of the catalogue is shared: change it in `myos-stacks`, not in place
+  on a server.
+- Before upgrading myos on a machine that already runs stacks, pin the naming
+  (`references/conventions.md`), or every project gets a new name.
+
+## More
+
+- `references/commands.md` — every command, and its equivalent in the old make targets
+- `references/conventions.md` — file layout, naming, networks, variables, tags
+- `references/troubleshooting.md` — what each failure means and how to fix it
+- `references/authoring.md` — writing a stack, and working on myos itself