feat: add confidence trace IDs to web-security reports

[GangGreenTemperTatum]

Summary

• generate a UUID trace ID inside web-security's assess_confidence output
• document copying the returned trace ID into report metadata
• cover the generated ID and schema behavior in Credence tests

Tests

• uv run pytest capabilities/web-security/tests/test_credence.py
• pre-commit run --files capabilities/web-security/tools/credence.py capabilities/web-security/tests/test_credence.py capabilities/web-security/skills/report-writer/SKILL.md
• just validate

Linear: CAP-1018