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

.github/workflows/codeql.yml

@@ -28,7 +28,6 @@ on:
       - "extensions/file-transfer/src/**"
       - "extensions/google-meet/src/**"
       - "extensions/imessage/src/**"
-      - "extensions/memory-core/src/memory/qmd-manager.ts"
       - "extensions/memory-wiki/src/obsidian.ts"
       - "extensions/microsoft-foundry/cli.ts"
       - "extensions/ollama/src/wsl2-crash-loop-check.ts"