Let a diary correction change who a memory was spoken to #133

Story

As a family member, I want to correct who a memory was spoken to, so that the diary shows it for the right person.

Context

A diary card records who a memo is addressed to (addressee). It comes from the per-message reading of the room (_read_room in stacklets/memory/bot/cli/diary.py), not from the card's own reading, and a correction in the card's thread cannot change it: diary_card.correct() leaves addressee as it was, and the correction prompt does not ask for it.

On the demo rig, one of Homer's memos is filed "Homer, for Homie": the reading took a nickname for the person spoken to. Replying "This was for Marge" updates the summary and facts, but the page heading keeps "for Homie".

Introduced with the diary cards in #132.

Proposal

Let a correction change the addressee the way it can change the date: the correction prompt returns an addressee when the correction states one, correct() applies it, and the card keeps it on every later compile (a corrected card is the family's).
Out of scope: changing how the per-message reading finds the addressee.

Definition of done

• A module test: correcting a card with "This was for Marge, not Homie" sets its addressee to Marge; a correction that says nothing about it leaves the addressee alone.
• On the rig, that reply in a card's thread gives the page heading "Homer, for Marge" and a correct: commit by the person who wrote it.
• docs/design/brain/vault-format.md (diary) says the addressee is correctable.