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

docs/docs.json

@@ -1260,7 +1260,6 @@
                       "concepts/user-model",
                       "concepts/standing-intents",
                       "concepts/memory-builtin",
-                      "concepts/memory-qmd",
                       "concepts/memory-honcho",
                       "concepts/memory-search",
                       "concepts/active-memory",