Summarize risk findings for review

[heznpc]

Summary

• add riskSummaries so human-facing reports group keyword risk findings by label
• keep full riskFindings in JSON for automation and --fail-on risk
• make corpus validation label risk_summary findings instead of every line-level risk match
• update validation/readme copy around risk-summary usefulness and underlying risk load

Real corpus check

• corpus: 64 real instruction documents, 51 roots, 13 holdout documents, 3,359 lines
• before risk summaries: 22 surface overlaps + 65 risk findings = 87 findings
• after risk summaries: 22 surface overlaps + 7 risk summaries = 29 findings
• with --experimental-similar: 52 total findings (adds 23 similar candidates)

Verification

• npm test
• npm run dogfood
• npm run validate:corpus
• node scripts/validate-corpus.mjs --manifest /tmp/rulemeter-real-corpus/manifest.json --format json --out /tmp/rulemeter-real-corpus/result.risk-summary.final.no-text.json
• node scripts/validate-corpus.mjs --manifest /tmp/rulemeter-real-corpus/manifest.json --format json --out /tmp/rulemeter-real-corpus/result.risk-summary-similar.final.no-text.json --experimental-similar
• npm run pack:check
• npm run smoke:install
• npm audit --audit-level=high