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/codeql/codeql-memory-runtime-boundary-critical-quality.yml

@@ -20,7 +20,6 @@ paths:
   - src/plugin-sdk/memory-*.ts
   - src/plugin-sdk/memory-core-host-*.ts
   - src/plugins/memory-*.ts
-  - src/gateway/server-startup-memory.ts
   - src/commands/doctor-memory-search.ts
   - src/commands/doctor/cron/dreaming-payload-migration.ts
 

.github/codeql/codeql-process-exec-boundary-critical-security.yml

@@ -31,7 +31,6 @@ paths:
   - 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