Pin actions to commit SHAs, set persist-credentials: false on every
checkout, and scope token permissions to the jobs that use them. Suppress
the two findings that are safe by design: the description bot's
pull_request_target trigger (no fork code runs) and an intentional
word-split in the docker manifest step.
Clears actionlint and zizmor against dev so the blocking gate from #1314
can pass once both land.
