fix(memory-core): frame the Kibitzer nudge as reference and make hints observations

The injected <recalled-memory> header told the primary agent to act ("verify
before relying on it; read the source path for full context") and the persona
let the sidecar write hints as instructions, so a nudge arriving in a context
without a real user task became the task. The header now names the sender and
the posture (reference only; the current task stands), and the persona's hint
rule, embedded block and examples ask for observations about the stored note
instead of directives. Persona and header both shrink.

Refs #8355