Adds test env delay incident report

[willik]

Summary

• Adds a polished incident report for the ~11-hour test environment delay on the muralmember-lastaccessdate env (mural-api PR #15619)
• Restructured from an investigation journal into a top-down report with front-loaded takeaways: summary, root cause, recommendations, detailed timeline, and appendices
• Root cause: force push image cycling created excess pods which, combined with cluster saturation and no autoscaler, left the API pod in FailedScheduling for 9.5 hours

Test plan

• Review document for factual accuracy
• Confirm all times are correct in UTC