Explain snapshot failures caused by regular directories

[hussainanjar]

When a backup restore leaves .snapshots as a regular directory, omarchy snapshot create stops with Snapper's opaque Btrfs error. Add guidance to locate the affected config's subvolume, preserve the directory's contents, and recreate it as a Btrfs subvolume before retrying.

Keep the original error and exit status, skip cleanup after failed creation, and leave unrelated failures without subvolume repair advice.

Fixes #11100.

Validation:

• Snapshot regression tests pass for root and non-root configs, original exit status, cleanup suppression, and unrelated failures. The new test failed before the fix.
• ./test/cli, Bash syntax checks, and git diff --check pass.
• Ran ./test/all (236 shell test files). Five environment/artifact failures passed on focused reruns after supplying the omarchy-pkgs and omarchy-iso checkouts, unsetting NO_COLOR, and removing generated Python bytecode.
• runtime-smoke-test.sh fails its duplicate IPC-handler check on this two-monitor session. The same check also fails on a clean upstream quattro checkout at b5589faa.