Persona notebooks: soft decay

[robert-dean]

Split from #25 §6, the only part of that discussion not yet built (the newsreader, #415, and the changeover break, #236, were the last two).

The notebook only grows. persona.notes.repository.ts rotates by last_used_at under PERSONA_NOTE_LIMITS, and the operator's reject and delete are the hard purge, but nothing decides what a character stops being. §6 argued the notebook wants soft decay specifically, and named three shapes:

• a half-life on last_used_at, no schema change, whose weakness is that it decays what the rotation happened not to pick;
• re-deriving a note from the scripts that used it;
• eviction on a signal about what did not land (break ratings, What the station's ratings of its own breaks are for #7, are the obvious source).

It also noted that an "already said, do not repeat" ban is not decay, and a decay rule must not quietly turn into one. The full argument is in #25 §6.