FEATURE: h8 set command — toggle container entrypoint

## Problem

No CLI command to change the container entrypoint. Needed for:
- Debug mode: `h8 set command myapp "sleep 3600"` to keep container alive for exec
- Restore: `h8 set command myapp --reset` to go back to Docker entrypoint

## Workaround

Raw API call:
```bash
curl -X PUT .../apps/xxx/ -d '{"command":"sleep 3600"}'
```

But this requires also including subdomain, SSL, disk, and envs to avoid wiping them (see #8).

## Proposal

```bash
h8 set command myapp "sleep 3600" # override entrypoint
h8 set command myapp --reset # back to Docker default
```