## Implementation Plan: Add `devplace containers prune-stopped` command

### Summary

Add a new CLI subcommand `prune-stopped` that:
1. Iterates all non‑deleted container instances.
2. For each instance with `status == ST_STOPPED` and `desired_state == DESIRED_STOPPED`, calls `api.mark_for_removal()`.
3. Then runs one full reconciliation pass (`ContainerService().run_once()`) so the marked instances are immediately removed via `backend.rm()` + `store.delete_instance()`.
4. Logs the number of containers marked for removal.

No changes to listing surfaces, the reconcile loop, or stored procedures are needed – the existing `ST_REMOVING` → removal pipeline handles everything after marking.

### Files to Modify

1. **`devplacepy/cli/containers.py`** (main implementation)  
   - Add `cmd_containers_prune_stopped(args)` function following the same pattern as `cmd_containers_prune`.  
   - Register it in `register_containers()` as subcommand `"prune-stopped"`.

2. **`devplacepy/cli/__init__.py`** (import registration)  
   - Add `from .containers import cmd_containers_prune_stopped` to `__all__` or the relevant import block.

3. **`devplacepy/templates/docs/services-containers.html`** (documentation)  
   - Add `prune-stopped` to the CLI usage line (e.g., `list | reconcile | prune | prune-builds | prune-stopped | gc-workspaces`).

### Detailed Code – `cmd_containers_prune_stopped` in `cli/containers.py`

```python
def cmd_containers_prune_stopped(args):
    """Remove all stopped containers that have completed their purpose."""
    async def run():
        from ..services.containers import store, api
        from ..services.containers.service import ContainerService

        instances = store.all_instances()
        marked = 0
        for inst in instances:
            if inst["status"] == store.ST_STOPPED and inst["desired_state"] == store.DESIRED_STOPPED:
                api.mark_for_removal(inst, actor=("system", "prune-stopped"))
                marked += 1

        if marked:
            await ContainerService().run_once()
            print(f"Prune stopped: marked {marked} container(s) for removal.")
        else:
            print("No stopped containers to prune.")
    async_runner(run())
```

**Registration** – inside `register_containers()` (near existing subcommand registrations):
```python
p = sub.add_parser("prune-stopped", help="Remove all stopped containers (completed purpose)")
p.set_defaults(func=cmd_containers_prune_stopped)
```

**Import in `cli/__init__.py`** – ensure `cmd_containers_prune_stopped` is exported (e.g., add to the existing tuple of command functions).

**Documentation update** – change the CLI usage line in `services-containers.html` from:
```
devplace containers list | reconcile | prune | prune-builds | gc-workspaces
```
to:
```
devplace containers list | reconcile | prune | prune-builds | prune-stopped | gc-workspaces
```

### Verification

1. **Existing tests pass**:  
   `pip install -e '.[dev]' -q && make test-unit`  (exit code 0)

2. **Linting succeeds**:  
   `pip install -q ruff && ruff check .` (exit code 0)

3. **New command is discoverable**:  
   `devplace containers --help` shows `prune-stopped`.

4. **Functional correctness** (manual runtime check, not required for CI but for acceptance):  
   - Start with at least one stopped container (`devplace containers list` shows status “stopped”).  
   - Run `devplace containers prune-stopped`.  
   - Confirm output reports the number of marked containers.  
   - After the next reconciliation cycle (or immediately because `run_once()` was called), the container no longer appears in `devplace containers list`.  
   - Disk space is freed (visible via `docker ps -a` or `df` on the store volume).

### Definition of Done

- [ ] `cmd_containers_prune_stopped` is implemented in `devplacepy/cli/containers.py`.
- [ ] Subcommand is registered in `register_containers()`.
- [ ] Import updated in `devplacepy/cli/__init__.py`.
- [ ] Documentation in `devplacepy/templates/docs/services-containers.html` lists the new subcommand.
- [ ] Running `devplace containers prune-stopped` with no arguments succeeds and only removes instances with `status == ST_STOPPED` and `desired_state == DESIRED_STOPPED`.
- [ ] The existing unit test suite (`make test-unit`) passes with no regressions.
- [ ] `ruff check .` passes with no new lint errors.