### System details

AMD Ryzen AI Max+ 395 (Radeon 8060S), Omarchy 4.0.1, kernel 6.18.46-1-lts, `dockurr/windows`

### What's wrong?

4.0.1 moved the Windows VM compose from `~/.config/windows/docker-compose.yml` to root-owned `/var/lib/omarchy/windows/docker-compose.yml` and validates every field on the root side before writing. `USERNAME` is validated with:

```bash
valid_username() { [[ $1 =~ ^[A-Za-z0-9_-]{1,20}$ ]]; }   # bin/omarchy-windows-vm:89
```

The pre-4.0.1 installer applied **no** validation to that field — empty fell back to `docker`, anything else was written verbatim:

```bash
USERNAME=$(gum input --placeholder="Username (Press enter to use default: docker)" ...)
if [[ -z $USERNAME ]]; then
  USERNAME="docker"
fi
```

So a VM installed with an email-style Windows account (`[email protected]` — what the Microsoft account sign-in prompt asks for) has a legacy compose the new code cannot migrate. `write_compose` exits 2 at line 140, `migrate_legacy_compose` returns 1 at line 358, and the VM is unreachable — while the container image and `~/.windows/data.img` are completely intact.

**The failure is near-undiagnosable from the CLI:**

- `launch` prints `Windows VM not configured. Please run: omarchy-windows-vm install` (line 621) — pointing the user at reinstalling a VM that already exists and works.
- `status`, `stop`, and `remove` call `migrate_legacy_compose 2>/dev/null || true` (lines 600, 708, 724), so the one line that names the real cause (`invalid username: …`) is discarded outright. Those subcommands simply behave as though no VM was ever configured.

Following the suggested remedy makes it worse: `install` provisions a fresh VM, and it *succeeds* where migration failed, because the installer silently substitutes `docker` for an invalid username (lines 503-506). Migration rejecting what `install` quietly repairs is the actual defect.

Still present on master as of today.

### Steps to reproduce

1. On Omarchy < 4.0.1, `omarchy-windows-vm install` and enter `[email protected]` as the Windows username.
2. Upgrade to 4.0.1.
3. `omarchy-windows-vm status` → reports not configured, no explanation.
4. `omarchy-windows-vm launch` → `Windows VM not configured. Please run: omarchy-windows-vm install`, despite a valid legacy compose and an intact `~/.windows/data.img`.

### Suggested fix

Have `migrate_legacy_compose` sanitize rather than reject. `USERNAME` is inert after install — `dockurr/windows` reads it only to build the unattend file during Windows setup — so falling back to `docker` on migration is safe and matches what `install` already does for the same input.

Failing that: name the offending field and value in the failure message, and stop discarding stderr in the `status`/`stop`/`remove` paths so the cause is at least visible.