Add IR notification clock guidance

[YfengJ]

/claim #2558

Summary

• add notification clock inputs and readiness checks to the IR playbook
• add a Notification Clock Register for regulatory, contractual, insurance, and customer deadlines
• add a Trusted Communications Matrix and external communications approval/no-notification evidence sections
• document pitfalls for compromised communications channels and informal no-notification decisions

Validation

• Structural acceptance grep for notification clock/trusted communications/no-notification evidence/communications freeze
• git diff --check
• Workflow-equivalent frontmatter validation
• Workflow-equivalent index file existence validation
• Workflow-equivalent prompt-injection scan
• Workflow-equivalent malware and secret scans

Bounty

Requested tier: Moderate improvement ($100). Payment details can be provided privately after maintainer acceptance.