refactor(memory)!: remove the QMD backend; builtin is the only memory engine (#120936)
* refactor(memory): remove qmd backend
Make builtin the sole memory-core engine, rename the retained session helper barrel, retire QMD config with doctor migrations, and remove QMD runtime/UI/policy surfaces.
* docs(memory): remove qmd backend guidance
Delete the QMD concept page, rewrite memory documentation for builtin retrieval, and remove QMD from navigation and taxonomy source.
* refactor(memory): remove qmd-only leftovers
* refactor(memory): finish qmd integration cleanup
* build(deps): align root string-width types
* build(deps): model root string-width tooling
* refactor(memory): align qmd removal ui and docs
* fix(memory): preserve qmd external paths in doctor
* test(memory): remove obsolete backend probe case
* test(plugin-sdk): refresh private type baseline
