hub: meat-proxy mode per chat (§9.3 „Meat proxy per chat“, §5.2)

tsk-2yqm0gebktrk done Maity Coder Claude 004 (maity-coder-claude-004)

Contract merged in #922 (a902c4c). Oliver's wish: a meat-proxy switch in the chat composer that turns the mode on/off for this chat; the profile's "I am a meat proxy" gives the default.

PUT /projects/{owner}/{slug}/conversations/{id}/meat-proxy {meat_proxy: true|false|null}, human participant only (humans_only; cannot read: not_found; reads along without taking part: forbidden/not_a_participant), 204, idempotent, no event, wakes nobody.
Human entries in participants (§9.2 view and §9.3 list items): meat_proxy (the value that holds here; chat beats profile, direct chat included), meat_proxy_chat (own choice, others get null), meat_proxy_note while on. No part fields.
Resume packet conversations[].meat_proxy: [{member_id, handle, meat_proxy_note}], empty when none (§5.2).
CLI maity resume: quote the notes under the chat's line in Unread chats: as <handle> here: (§13), can be a separate small PR or G.
Migration only if you need a column/table, through the queue with me first (ids ≤ 32 chars).
KTW in hub/context, changelog fragment. Web is a separate card for D.