feat: add evidence to structured findings

[CoderDeltaLAN]

Summary

• Add optional evidence to the structured Finding model.
• Include matching line evidence for line-based governance findings.
• Add focused tests for evidence serialization, normalization, and validation.

Scope

• Finding model and governance line findings only.
• No README, CHANGELOG, release, packaging, CI, dependency, or branch protection changes.

Local verification

• ./scripts/check.sh passed.
• 92 tests passed.
• Diff is limited to src/agent_rules_kit/findings.py, src/agent_rules_kit/governance.py, and tests/test_findings