React to agent replies (thumbs up/down with a note) so feedback reaches the agent

[aien-spark]

Today the only way to tell an agent "that reply was wrong" is to type a message. A lightweight reaction on each assistant message would make feedback cheap and keep it attached to the reply it's about.

Proposed behaviour, kept small:

• Thumbs-up / thumbs-down in the assistant message hover toolbar (web, desktop). Thumbs-down opens an optional one-line note. Click again to clear. Mobile shows the reaction read-only for now.
• The reaction is stored on the assistant message item via a new message.react command, so it survives restarts and shows on every device.
• Delivery reuses composer context records: reactions made since the user's last message ride along on the next message as a message-reaction record, rendered to the agent as a short <user_feedback> block ("The user gave a thumbs-down to your earlier reply that began '…'. Reason: … If this points to a pattern, record it in your memory or instructions."). No per-provider work; old servers drop the record harmlessly.
• Deliberately out of scope: steering the running turn immediately, and reacting from mobile.

I have a working branch with tests at the agreed seams (orchestrator command, prompt wording, bundle selection). I'll add screenshots in a comment below. Happy to adjust the shape before opening a PR, or to drop it if this doesn't fit the roadmap.

[aien-spark]

Screenshots from the working branch (aien-dev/t3code → feat/message-reactions, compare: main...aien-dev:t3code:feat/message-reactions):

Thumbs-down opens a one-line note box in the reply's hover toolbar:

The chosen thumb stays lit; hovering shows the note, clicking again clears it:

The next user message carries the reaction; here the agent (Codex) quotes the note back verbatim: