[MB-0606][P2] Add operational runbooks for migration, rollback, and incidents

[FarisZR]

Summary

Add operations runbooks for safe rollout, rollback, migration verification, and incident handling.

Scope

• Create runbooks for deployment verification, rollback, data migration validation, and incident triage.
• Link runbooks from docs index/README.

Acceptance Criteria

• Operators can execute deploy/rollback/migration validation from docs without implicit knowledge.
• Incident triage path includes clear decision points and escalation notes.

Test Requirements (Mandatory)

• Tabletop run-through completed and attached as evidence in PR/issue comments.

Dependencies

• MB-0404
• MB-0605

Definition of Done

• Runbooks merged and cross-linked.
• Tabletop exercise evidence recorded.

References

• Plan: docs/plans/2026-02-21-refactor-long-term-modernization-backlog-plan.md
• Backlog ID: MB-0606