Security Hub collector: export CSPM findings

[MattDonnellySoftrams]

Summary

• Add a read-only boto3 Security Hub/CSPM findings collector.
• Support profile/region selection, severity and workflow filters, pagination, and bounded exports.
• Normalize findings with stable deduplication keys and AWS console links for later Jira processing.

Validation

• python3 -m py_compile scripts/securityhub_collect.py
• git diff --check