S-1: Registry Governance Documentation

## Problem / User Value

Partner teams at Microsoft need a clear, documented process for submitting their tools to the azd registry. Without a governance model, the registry becomes ungoverned — either no one knows how to contribute, or tools are added without quality review, leading to stale or broken entries.

## Definition of Done

Governance document published at `docs/design/azd-tool/registry-governance.md` with ownership, approval process, SLA, contribution template, and ingestion process.

## Tasks

- [ ] **S-1: Write registry governance documentation** (S) — Create `docs/design/azd-tool/registry-governance.md` documenting: registry ownership (azd team), approval process (PR review by azd maintainers), SLA for additions (<5 business days), ingestion model (PR with YAML entry + test validation), contribution template with all required fields. Reference `pkg/extensions/registry.go` as prior art for the extension registry model.
- AC: Governance doc published; ownership/approval/SLA clearly defined; contribution template included with all required fields; ingestion process described (PR-based with CI validation); document reviewed and approved by team

## Sequencing

No dependencies — can start immediately and run in parallel with any other work.

## Dependencies

None. This is a standalone documentation task with no code dependencies.

## Discussion Traceability

| Discussion Item | Task |
|---|---|
| S1 (registry curation) | S-1 |

## Design References

- `docs/design/azd-tool/plan.md` — Standalone Tasks
- `docs/design/azd-tool/todos.md` — S-1 acceptance criteria
- `pkg/extensions/registry.go` — prior art for extension registry model