fix ruff failures

[hendo-21]

Approach:

• Used pre-commit run --all-files to address all but 4 ruff check errors. pre-commit resolved ruff format failure.
• Manually applied fixes for code style violations that Ruff flags as unsafe for auto-fixing in the following files:
  • src/fenn/agents/__init__.py: suppressed E402 error in-line with noqa since import placement appeared intentional to avoid circular import
  • src/fenn/agents/node.py: changed variable names so no longer ambiguous
  • src/fenn/agents/rag/retriever.py: suppressed F401 error in-line with noqa since numpy import seems required for cofone dependency check

[blkdmr]

Thank you very much for your work!