The adhoc workflow checked out any requested ref and ran its scripts and
electron-builder config with MAC_CERTS, the notary password, and the adhoc
publisher token in reach — including refs/pull/* fork code a maintainer
could dispatch in one innocuous-looking click.
Vet the ref before checkout: PR refs are refused, branches/tags resolve in
a bare tree:0 scratch fetch, raw SHAs must be reachable from a repo branch
or tag (a partial clone lazily serves PR-only commits by SHA, so name
resolution alone is not a trust test), and checkout pins the vetted SHA so
a race push cannot swap the commit. Also reference an adhoc-mac-build
environment so the secrets can later be fenced off from stale workflow
copies via repo settings.
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