Define secure decommissioning process for production agents

## Description

No current item defines what happens when a production agent is retired.

## Source

Krav 10 "Sikker avvikling": access/keys revoked, sub-agents and scheduled tasks stopped, logs archived, registry status updated.

## Business Goal

Prevents "zombie" agents retaining live credentials or scheduled tasks after a team considers them retired — a direct security and accountability risk.

## Acceptance Criteria

- Documented decommissioning checklist covering: access/key revocation, sub-agent and scheduled-task termination, log archival, and registry status update to "decommissioned" with a date.
- Checklist is referenced from the agent registry (#75) as a required step, not optional guidance.

## Dependencies

Builds on #75 (registry status field).