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

scripts/lib/plugin-sdk-entrypoints.json

@@ -259,7 +259,7 @@
   "memory-core-host-engine-curated",
   "memory-core-host-engine-foundation",
   "memory-core-host-engine-fs",
-  "memory-core-host-engine-qmd",
+  "memory-core-host-engine-sessions",
   "memory-core-host-engine-schema",
   "memory-core-host-engine-storage",
   "memory-core-host-secret",

scripts/lib/plugin-sdk-private-local-only-subpaths.json

@@ -67,7 +67,7 @@
   "memory-core-host-engine-curated",
   "memory-core-host-engine-embeddings",
   "memory-core-host-engine-fs",
-  "memory-core-host-engine-qmd",
+  "memory-core-host-engine-sessions",
   "memory-core-host-engine-schema",
   "memory-core-host-engine-storage",
   "memory-core-host-runtime-cli",

scripts/lib/policy-config-coverage.jsonc

@@ -28,7 +28,6 @@
     "gateway.nodes.commands.deny.*",
     "gateway.tailscale.mode",
     "gateway.tailscale.preserveFunnel",
-    "memory.qmd.sessions.enabled",
     "mcp.servers.*.command",
     "mcp.servers.*.transport",
     "mcp.servers.*.url",
@@ -187,13 +186,6 @@
       "policy": "dataHandling.retention.requireSessionMaintenance",
       "reason": "Policy observes session maintenance enforcement posture.",
     },
-    {
-      "pattern": "memory.qmd.sessions.enabled",
-      "status": "observed",
-      "area": "dataHandling",
-      "policy": "dataHandling.memory.denySessionTranscriptIndexing",
-      "reason": "Policy observes QMD session-transcript indexing.",
-    },
     {
       "pattern": "memory.search.enabled",
       "status": "observed",