Sign commits and bump create-pull-request to v7 in black-format workflow

[claude]

Requested by Ole Herman · Slack thread

Updates the auto-formatting workflow's pull-request step.

Before: .github/workflows/black-format.yml ran the cfengine/create-pull-request fork at @v6, which does not support commit signing, so the bot's auto-generated formatting commits were unsigned.

After: it runs @v7 with sign-commits: true, so the commits the action creates are signed.

Notes:

• v6 -> v7 is a major version bump (the action moved to Node 20).
• Commit signing requires the job token to have contents: write, which this job's permissions: block already grants.
• Three sibling repos (build-website, documentation, website) already run this exact combo (@v7 + sign-commits: true).

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