ci: add black auto-format workflow (#2827)

[priyank766]

Closes #2827

Adds a GitHub Actions workflow (black-format.yml) that automatically runs black and isort on dependabot PRs, removing the need to manually format code after dependency updates.

How it works:

• Triggers on pull_request (only runs if the actor is dependabot[bot]) or workflow_dispatch (manual trigger)
• Runs isort and black with the same configuration used in .pre-commit-config.yaml:
  • --line-length 120
  • Excludes *_pb2.py files
  • Targets capa/, scripts/, tests/, web/rules/scripts/
• If any files change, it commits and pushes them back to the PR branch automatically

Checklist

• No CHANGELOG update needed
• No new tests needed
• No documentation update needed
• This submission includes AI-generated code and I have provided details in the description.

[gemini-code-assist]

Note

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[priyank766]

@williballenthin