Improve bot contributor filtering

[beanbean9339]

Update the contributor filtering logic to more reliably exclude bot-generated contributors from imported GitHub metadata.

Currently, we maintain a list of known Claude bot accounts, but GitHub/Copilot-related bot names can vary. Instead of relying only on specific account names, the filtering logic should also exclude contributors whose username or name contains "copilot".

Proposed Changes

• Add a case-insensitive check for "copilot" when filtering GitHub contributors.
• Continue excluding known Claude bot accounts using the existing exclusion list.
• Ensure bot accounts are not included in generated contributor metadata such as CITATION.cff or .zenodo.json.
• Add or update tests covering:
  • Contributors with "copilot" in their name or username
  • Known Claude bot accounts
  • Normal human contributors whose names do not match the bot filters

Acceptance Criteria

• Contributors containing "copilot" in their username or name are excluded.
• Known Claude bot accounts continue to be excluded.
• Human contributors are not unintentionally filtered out.
• Tests are added or updated for the new filtering behavior.
• Existing GitHub contributor import functionality continues to pass all tests.

Context

This came up during review of PR #48, where we noticed that bot contributor names have been changing ("whack-a-mole"). This issue tracks improving the filtering logic so it is more robust to future Copilot bot naming variations.

This is not a blocker for PR #48.