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