### What happened

`omarchy-update-available` has the same unguarded `$OMARCHY_PATH` dereference that #8369 reports in `omarchy-update-dev`, and #8590 only fixes the latter. With `OMARCHY_PATH` unset it dies on line 9:

```
$ env -u OMARCHY_PATH /usr/bin/omarchy-update-available
/usr/bin/omarchy-update-available: line 9: OMARCHY_PATH: unbound variable
```

### Why this one matters separately

`omarchy-update-status` runs it near the end of **every** `omarchy update`:

```bash
if omarchy-update-available >/dev/null; then
  omarchy-shell -q omarchy.system-update refresh
else
  omarchy-shell -q omarchy.system-update clear
fi
```

So on the `sudo omarchy update` path from #8369, landing #8590 alone moves the crash rather than removing it: the update gets past `omarchy-update-dev` and then hits the identical error here.

Worse, this one fails **silently**. `omarchy-update-available` exits 1 both when Omarchy is up to date and when it crashes, and `omarchy-update-status` consumes that status in an `if`. A crash therefore takes the `clear` branch — the bar's update indicator is cleared as though the system were up to date, and because only stdout is redirected the sole trace is a stray `unbound variable` line in the update output.

### Cause

```bash
set -euo pipefail

updates=()

if [[ $OMARCHY_PATH != "/usr/share/omarchy" ]]; then
```

Same shape as #8369: `set -u` plus a bare `$OMARCHY_PATH` on the very check meant to recognize a package-backed install. Any context that never sourced `default/bash/rc` — a `sudo`'d update under the default `env_reset`, a non-login SSH session — reaches it unset.

Note that #7398 also touches this file, but it rewrites the comparison as `${OMARCHY_PATH%/}`, which is still a bare dereference under `set -u`, so it does not fix this either.

### Expected

The fallback the rest of the codebase uses (`default/bash/rc`, `default/bash/env-bootstrap`, `bin/omarchy-migrate`, `bin/omarchy-version`):

```bash
: "${OMARCHY_PATH:=/usr/share/omarchy}"
```

An unset `OMARCHY_PATH` should classify the install as package-backed and skip the dev-checkout check, which is exactly what it means.

### System

- Omarchy 4.0.0-1 (`/usr/share/omarchy/version` reports 4.0.0.alpha)
- Kernel 7.1.8-arch1-3
- sudoers: default `env_reset`, no `OMARCHY_PATH` in `env_keep`