Postmortem template

Canonical template for the Postmortem format.

# Postmortem: [Incident Title]

## Severity
[SEV-1 | SEV-2 | SEV-3 | SEV-4]

## Summary
[2-3 sentences: what broke, when, and the blast radius.]

## Timeline
- HH:MM UTC - [event]
- HH:MM UTC - [event]

## Root Cause
[The systemic condition that allowed the incident to occur. Follow the chain past the proximate trigger.]

## Impact
- Users affected: [count or percentage]
- Duration: [start] to [end]
- Services affected: [list]

## Contributing Factors
- [factor]

## Action Items
- [ ] [Specific change] - Owner: [person/team] - Due: [date]

## Lessons Learned
[What the team now knows that it did not know before.]