Reset any stash changes before proceeding if there were conflicts

If the worktree has conflicts after applying the user's changes from the
autostash, we should reset them before proceeding to ensure we are in
a working state. When there are conflicts, git still keeps the stash
entry, so the user will still be able to manually re-pop the stash and
resolve the conflicts after `omarchy-update` has finished.

In the case of conflicts, the output will look something like this (I've
omitted most of the normal `git pull` output, hence the `[...]`):

```
Updating 729cd6a..45b5d3e
Created autostash: 91853c4
Fast-forward
 bin/omarchy                                                                |  10 +++++++---
 [...]
 create mode 100644 themes/tokyo-night/backgrounds/2--Milad-Fakurian-Abstract-Purple-Blue.jpg
Applying autostash resulted in conflicts.
Your changes are safe in the stash.
You can run "git stash pop" or "git stash drop" at any time.
bin/omarchy:65: leftover conflict marker
```

before proceeding with the rest of the `omarchy-update` script from a
clean state. So the user will see 1) that there were conflicts when
applying the autostashed changes, 2) that their changes are still safe
in the stash, and 3) which files (and lines) had conflict markers.

bin/omarchy-update

@@ -12,6 +12,7 @@ fi
 
 # Get the latest while trying to preserve any modifications
 git pull --autostash
+git diff --check || git reset --merge
 
 # Run any pending migrations
 for file in migrations/*.sh; do