Improve patch exception revalidation gates

[danyili2632]

Summary

• adds trigger-based revalidation gates for deferred vulnerability exceptions
• covers vendor patch/advisory availability, CISA KEV, EPSS movement, public/active exploitation, exposure/criticality changes, scanner evidence changes, and compensating control drift
• adds required report output fields for last/next revalidation, triggers checked, SSVC/SLA change, control retest, and resulting action
• adds a common pitfall warning against waiting for fixed review dates after conditions change

Validation

• git diff --check
• rg -n "Deferred Vulnerability Revalidation|Vendor patch|CISA KEV|EPSS|public exploit|Asset exposure|Compensating control retest|Last revalidation|Next revalidation|fixed review date" skills/vuln-management/patch-prioritization/SKILL.md
• verified Markdown fence count remains even: 12

Closes #1401

Bounty target: Improver Moderate if accepted.
Preferred payout: Base USDC 0x6CBF4b5cb88b8C2B7af776Bc2B073163B5d3C08A