Add CI validation harness using Github Actions.

[folone]

Adds some basic linters/checkers to make it easier to sanity-check contributions.

Note: shellcheck had a bunch of complaints about the existing migrations. I've chosen to ignore these by filtering out everything before the latest migration 1754228679 inclusively. Meaning, new migrations will run through the shellcheck.

The rest of the changes to the migrations files is just making them executable and replacing ~ with $HOME.

See an example of this workflow running in my fork here folone#1

[dhh]

Thanks for investigating this! I'd actually very much like a CI system, but I don't want it to be a linter, I want it to be a tester. None of the checks added here are something I've seen be an issue in either Omarchy or Omakub. But if you can find a way to actually run a complete install of Omarchy and verify that it boots correctly, I'm very interested in that. Please do open another PR if you get there!

[folone]

if you can find a way to actually run a complete install of Omarchy and verify that it boots correctly

Good idea – let me see if we can do something with containers 🤔