fix(dashboard): classify memory.external in ConfigEditor coverage manifest

The external-memory-backend branch added the memory.external schema subtree
(provider/endpoint/banks/retain_sources/tags + recall sub-block) but did not
add a corresponding entry to the dashboard's config-field-coverage manifest.
The config-parity guard (config-parity.test.ts) enforces that every schema
leaf is either RENDERED by the ConfigEditor form or listed in
OMITTED_BY_DESIGN, so CI failed with 16 uncovered leaves.

memory.external is USER-config-only and operator-configured (no form widgets
exist for it), so the correct classification is OMITTED_BY_DESIGN with the
whole subtree covered by the 'memory.external' prefix.

Author: codeo1io

packages/dashboard/src/components/ConfigEditor/config-field-coverage.ts

@@ -83,6 +83,11 @@ export const OMITTED_BY_DESIGN: Readonly<Record<string, string>> = {
   "sidekick.system_prompt": "free-form prompt override; raw JSONC",
   "system_prompt_injection.skip_signatures":
     "free-form substring array; raw JSONC (no array widget in the form yet)",
+  // External memory backend (Hindsight tee + recall). USER config only and
+  // operator-configured (endpoint/api_key, bank routing, recall tuning); no
+  // form widgets exist. Whole subtree is raw-JSONC by design.
+  "memory.external":
+    "external long-term memory backend (provider/endpoint/banks/retain_sources/tags + recall sub-block); USER config only, operator-configured, raw JSONC",
 };
 
 /**