feat(ci): add CI workflow

Format check and build on Arch container

Signed-off-by: Josephine Pfeiffer <hi@josie.lol>

# Please enter the commit message for your changes. Lines starting
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# Date:      Fri Feb 20 16:58:36 2026 +0100
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# On branch feat/add-ci
# Changes to be committed:
#	new file:   .github/workflows/aur-fetch.yml
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