Add passport number PII detection pattern

[Acacian]

Summary

The PII guardrail currently detects 12 categories of sensitive data, but passport numbers only match when a keyword like "passport" appears nearby. Add a standalone regex pattern for common passport formats.

What to do

1. Open src/aegis/guardrails/pii.py
2. Add regex patterns for common passport formats:
   • US: 1 letter + 8 digits (e.g., A12345678)
   • UK: 9 digits
   • Korean: 1-2 letters + 7 digits
3. Add tests in tests/test_guardrails_pii.py
4. Run pytest tests/test_guardrails_pii.py -x to verify

Context

The existing patterns live in the PIIGuardrail class. Look at how email or ssn patterns are defined for the pattern to follow.

Acceptance criteria

• New pattern matches common passport formats
• Existing tests still pass
• At least 3 new test cases added