feat(hub): Model Registry client module

Implement the `hub` client module wrapping `kubeflow.hub.ModelRegistryClient` (SDK 0.4.x). Completes the train -> register -> serve lifecycle within a single MCP session.

## Context

- SDK 0.4.0 shipped `ModelRegistryClient` (v0.3.0+)
- Stub exists at `kubeflow_mcp/hub/`
- Without this, agents hit a dead end after "job complete"

## Proposed Tools

Read-only:
- `list_models`, `get_model`, `list_model_versions`, `get_model_version`, `compare_models`

Mutating (confirm gate):
- `register_model`, `promote_model`, `update_model_metadata`

## Acceptance Criteria

- Follows module contract (TOOLS, CLIENT_TOOL_DESCRIPTIONS, CLIENT_TOOL_ANNOTATIONS, INSTRUCTION_SECTIONS, CLIENT_RESOURCES, SECTION_ORDER)
- Confirm gate on mutations, persona filtering consistent with trainer
- Co-located tests, schema snapshot updated
- Resource: `hub://guides/model-lifecycle`

## References

- SDK source: `kubeflow/hub/api/model_registry_client.py`
- ROADMAP Phase 6
- Related: kubeflow/sdk#107