Define logging & traceability requirements, incl. shared correlation ID across agents

## Description

No current backlog item defines what must be logged for an agent's actions, or how to reconstruct a request's path through a multi-agent system.

## Source

Krav 7 "Sporbarhet og logging" and Krav 14 "Sporbarhet på tvers av agenter".

## Business Goal

Logging is the basis for incident response, audits, and public-sector transparency obligations (forvaltningsloven, offentleglova). Without it, "what happened" is unanswerable after an incident.

## Acceptance Criteria

- Documented minimum logging requirements — instructions received, tools/actions used, decisions made, outcomes — scaled by agent class.
- A shared correlation-ID format is specified for multi-agent/orchestrator systems, propagated from the initial request to the final output.
- Guidance on log retention/access aligned with existing Digdir logging infrastructure.

## Dependencies

Informs the approval-gate issue (approval events should be logged) and the mandate-delegation-limits issue (delegation should be auditable).