Ci/GitHub actions tests

[Mina-Sayed]

Summary

This PR adds GitHub Actions CI for Omarchy’s existing test suites.

The workflow:

• runs on pull requests and pushes targeting quattro
• executes the existing test/cli and test/shell suites as separate matrix jobs
• uses an Arch Linux base-devel container on an Ubuntu 24.04 runner
• checks out omarchy-pkgs and omarchy-iso alongside Omarchy
• runs tests as an unprivileged user
• pins actions/checkout to a full commit SHA
• uses read-only repository permissions
• cancels outdated runs for the same branch
• supports manual execution through workflow_dispatch

Architecture

The initial CI intentionally focuses on fast, deterministic validation:

• CLI suite: command metadata and CLI behavior
• Shell suite: shell scripts, installers, migrations, and desktop behavior
• Separate matrix jobs: failures are isolated and both suites remain visible
• Arch container: matches Omarchy’s target distribution more closely than the runner host
• Sibling repositories: preserves the directory layout expected by the current tests

Full ISO builds and QEMU installation tests are intentionally excluded from pull-request CI. They are better suited to a separate scheduled or manually triggered acceptance workflow because they require more time and compute.

Security

• workflow permissions are limited to contents: read
• checkout credentials are not persisted
• third-party actions are pinned to immutable commit SHAs
• repository tests run as an unprivileged ci user

Documentation

Adds docs/testing.md covering:

• local test execution
• required sibling repository layout
• GitHub Actions behavior
• the boundary between fast CI and future ISO/QEMU acceptance testing

Verification

Validated on the fork with both matrix jobs passing:

• cli ✅
• shell ✅

CI run: https://github.com/Mina-Sayed/omarchy/actions/runs/33875551298