Add Fedora CI checks, tests, and sourceable guard for uki-setup.sh

[GlitchSlayed]

Motivation

• Add reproducible CI checks in a Fedora container to validate the installer script and prevent regressions.
• Make the main setup script safe to source from tests/CI so non-destructive validation can run without performing system changes.

Description

• Add a GitHub Actions workflow at .github/workflows/checks.yml that runs inside fedora:41, installs check dependencies, and runs Bash syntax checks, shellcheck, and project-level tests.
• Guard the script execution path in uki-setup.sh behind UKI_SETUP_SKIP_MAIN, so tests can source the file without triggering destructive phases.
• Add tests/test_uki_setup.sh which sources uki-setup.sh with UKI_SETUP_SKIP_MAIN=1, runs the template-generation phases into a temporary workspace, and asserts the helper/plugin files are created, executable, and have placeholders substituted.
• Update README.md with a short section documenting the CI checks and the equivalent local commands to run the same validations.

Testing

• bash -n uki-setup.sh tests/test_uki_setup.sh was executed and returned successfully.
• bash tests/test_uki_setup.sh was executed locally and completed successfully, asserting generated files and substitutions.
• shellcheck uki-setup.sh tests/test_uki_setup.sh could not be run in this runtime because shellcheck was not installed here, but the workflow installs and enforces shellcheck in CI.

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Codex Task