Use new spaceUsage endpoint in place of deprecated spaceStatus

[murrayju]

Updates the CLI to call the new GET /spaces/{space_id}/usage endpoint introduced in timescale/ghost-api#16, replacing the now-deprecated GET /spaces/{space_id}/status endpoint. Also renames the internal helper to match.

Changes

• openapi.yaml — synced from ghost-api to pick up the new spaceUsage operation. The deprecated spaceStatus operation remains as an alias.
• internal/api/{client,types}.go, internal/api/mock/mock_client.go — regenerated via go generate ./....
• internal/common/{status.go → usage.go} — file renamed; FetchStatus → FetchUsage, Status struct → Usage. Now calls SpaceUsageWithResponse and reads *api.SpaceUsage.
• internal/cmd/usage.go, internal/mcp/usage.go — updated callers and local variable names.
• internal/cmd/usage_test.go — mock expectations and test case names updated.

Verification

./check passes.

[nathanjcochran]

Broadly LGTM, though I think there a few more internal things we should rename as well