feat: pos docker stack — containers grouped by compose stack (project)

New bin/pos-docker-stack: docker ps output grouped by Docker Compose
project (stack). Each stack is a sorted section (name, status, ports);
containers without a compose project land in a Standalone group at the
end. Running only by default, -a|--all includes stopped/exited. Status
colored on a terminal; summary line 'Stacks: N containers: N
standalone: N'; exit 0 when empty.

Data via docker ps --format with \x1f delimiters (project label
com.docker.compose.project from compose v2); parsed with awk -F'\x1f'
+ IFS=$'\x1f' read — tab/pipe delimiters are IFS whitespace or appear
in values (DEV.md:213). Dash padding via sed, not tr (multi-byte).
Deps guard (docker) before --help; no stdin.

Docs: POS.md docker row + detail, howto/docker.md table + section,
bin/pos usage EXAMPLES, AGENT_Context Common Tasks row. Verified:
stub suite 23/23, live daemon runs, dispatch, make gen && make check,
make lint 0 FAIL / 0 WARN.

DOC/howto/docker.md

@@ -1,13 +1,14 @@
 # How-To: `pos docker`
 
 Manage self-hosted services, watch containers, and spin up disposable VMs.
-Tools: `compose`, `ps`, `health`, `vbox`.
+Tools: `compose`, `ps`, `health`, `stack`, `vbox`.
 
 | Tool | What it does |
 |------|--------------|
 | `pos docker compose` | ScaleTail service manager (deploy/stop/logs/update) |
 | `pos docker ps` | Enhanced container overview (health, IPs, ports, uptime) |
 | `pos docker health` | One-glance health dashboard (exits 1 if unhealthy) |
+| `pos docker stack` | Containers grouped by compose stack (`-a` includes stopped) |
 | `pos docker vbox` | Disposable Docker containers as lightweight VMs |
 
 ---
@@ -100,6 +101,31 @@ not unhealthy.
 
 ---
 
+## `pos docker stack` — containers by compose stack
+
+```bash
+pos docker stack          # running containers grouped by compose project
+pos docker stack -a       # include stopped/exited containers
+```
+
+Grouped view of your compose deployments: every stack is a section with its
+services (container name, status, ports); containers started outside compose
+land in a `Standalone` group at the end. `-a` behaves like `docker ps -a` and
+also shows exited services (e.g. one-shot migration jobs that `docker ps`
+hides).
+
+**Recipe:** after `pos docker compose up <svc>`, confirm it joined the right
+stack:
+
+```bash
+pos docker stack | grep -A5 <svc>
+```
+
+**Recipe:** spot what's restarting across all stacks at a glance (yellow
+`Restarting` / red `Exited` statuses stand out on a terminal).
+
+---
+
 ## `pos docker vbox` — disposable VMs
 
 Docker containers with a bind-mounted host dir so files persist even after the