feat(memory): enforce English-only memory indexing with CJK guardrail

Memory layer is now English-indexed: memories() query terms and
  remember() summaries must be English. Adds a runtime assertion that
  rejects CJK text in both paths, guiding the agent to translate
  non-English user requests before querying or writing memories.
  Ensures consistent retrieval regardless of conversation language.

references/retrieval-semantics.md

@@ -106,6 +106,11 @@ For semantic questions, build a task-local evidence view:
 }
 ```
 
+Memory recall is English-indexed: translate non-English user requests into
+concise English query terms before calling `memories({ query })`. Memory
+summaries registered with `remember()` are also English, regardless of the
+conversation language.
+
 Then synthesize the conclusion in the final answer. Do not pretend the raw
 evidence view is itself a stored Obelisk entity.