platform_management capability retired in favor of platform

Built-in capabilities now have a two-step removal lifecycle. A deprecated capability keeps working while catalogs warn. A retired one stays registered so agents that reference it still start, but it contributes no tools, prompt, mounts or hooks, and is hidden from catalogs unless include_retired is set.

platform_management is retired. Its 14 hand-written tools are replaced by the platform capability, which derives the same surface from the command catalog. Agents that still list platform_management should switch to platform.

See Platform Management (removed) and Platform. #3513