docs: add PIR remediation verification gates

[catcherintheroad-hub]

Created from review issue: #1398

Summary

• Add remediation verification and recurrence monitoring gates to post-incident-review
• Require acceptance criteria, implementation evidence, independent validation, detection validation, closure approval, and residual risk handling before remediation closure
• Add edge-case fixtures for ticket-only closure, untested detection rules, backup changes without restore testing, and complete closure evidence

Validation

• git diff --check
• Markdown fence balance and ASCII check for touched files
• Reference URL checks for NIST SP 800-61 Rev. 2, NIST Cybersecurity Framework, and Google SRE Postmortem Culture