Pin CI actions and add zizmor and dependabot

[andrew]

Pins actions/checkout and actions/setup-node to commit SHAs, sets permissions: {} at the top level with contents: read on the job, and passes persist-credentials: false to checkout. Adds a zizmor workflow and a dependabot config for github-actions and npm with a 7 day cooldown.

Also adds Node 24 to the test matrix. Node 18 stays while engines is >=18.

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