test(memory-core): pin the nudge-only hint contract and the persona sample block

Kibitzer hints are reference material about a stored note, not instructions to
the primary agent, and 54% of delivered hints were still phrased as orders
(#8355). These tests state the boundary contract before it exists:
describeInvalidHint names "addresses-agent" for second-person, imperative-opening
and Korean request-ending hints, validateNudges drops them without reserving the
path, and observational hints - including ones carrying must/never mid-sentence
and Korean plain-form endings - survive both layers.

isValidHint stays the replay predicate: a nudge accepted before this contract
existed must keep rendering and keep being delivered, so only admission applies
the new rule.

render.test.ts pins persona and renderer to one source: the fenced
<recalled-memory> sample in kibitzer-persona.md must equal renderNudgeBlock's
output byte for byte.