The per-image jobs in deploy.yml were gated on
contains(github.event.head_commit.modified, …). That list only describes
the tip commit of a push and Gitea leaves it empty for merge commits, so
every PR landed with a merge commit skipped all six image builds; the
llm image for #1120 had to be built and deployed by hand.
A new 'changes' job fetches the push's previous tip (github.event.before)
into the shallow checkout and diffs it against the new tip, so the whole
pushed range counts: a merge commit's branch, or all commits of a direct
multi-commit push. Each image job now needs that job and gates on its
boolean output; an image without path filters follows every push. When
the previous tip is unknown or unfetchable (first push, history rewrite)
every image builds rather than none.
Tests cover the generated shape and run the detector script in a scratch
repository for the merge-commit, multi-commit and unknown-tip cases.
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