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

extensions/qa-lab/src/live-transports/shared/live-gateway.runtime.test.ts

@@ -146,7 +146,7 @@ describe("startQaLiveLaneGateway", () => {
         },
         slots: {
           memory: "memory-core",
-          contextEngine: "qmd",
+          contextEngine: "custom-context",
         },
       },
       memory: {
@@ -163,7 +163,7 @@ describe("startQaLiveLaneGateway", () => {
     expect(cfg?.plugins?.allow).toEqual(["acpx", "qa-channel"]);
     expect(cfg?.plugins?.entries).not.toHaveProperty("memory-core");
     expect(cfg?.plugins?.slots?.memory).toBe("none");
-    expect(cfg?.plugins?.slots?.contextEngine).toBe("qmd");
+    expect(cfg?.plugins?.slots?.contextEngine).toBe("custom-context");
     expect(cfg?.memory?.search?.enabled).toBe(false);
   });
 

extensions/qa-lab/src/qa-gateway-config.ts

@@ -218,7 +218,6 @@ export function buildQaGatewayConfig(params: {
       lastTouchedVersion: OPENCLAW_VERSION,
     },
     memory: {
-      backend: "builtin",
       search: {
         ...mockMemorySearch,
       },