Implement the Koios governance data provider

## Outcome

The Governance API can serve its committed governance data contract through Koios.

## Scope

- Map Koios responses to canonical Governance API types
- Implement provider health and capability reporting
- Handle rate limits, pagination, retries, and provider-specific errors
- Pass the shared provider contract tests

## Acceptance criteria

- [ ] Scope is decomposed into independently deliverable child issues.
- [ ] Architecture and API decisions are linked from this issue.
- [ ] Test, documentation, observability, rollout, and failure behavior are defined where applicable.
- [ ] Epic exit criteria are verified before closure.

## Planning note

This is a 2026 GovTool revamp work item. Its child features and tasks should be added as the scope is refined.