feat(chat): emoji reactions on Chat messages, both directions

[selfcontained]

Summary

Emoji reactions in the Chat tab, in both directions.

User → agent. Hover (or, on touch, always see) the smiley action on an agent post to open a picker of 12 emoji. The reaction shows as a chip under the post and is injected into the agent's pane — through the same quiet-gated delivery as a user message — so the chip carries delivery state (dimmed while pending, ⚠ when undelivered). Tap your chip to take it back; removal is display-only.

Agent → user. Agents react to the user's posts with the new dispatch_chat_react MCP tool ({ messageId, emoji, remove? }). These render as neutral chips the user can't remove and are never injected anywhere.

Reaction envelope

Identifies the post without pasting it back:

--- DISPATCH CHAT REACTION (message id: <id>) ---
The user reacted 🎉 to your summary from 3 posts ago:
> All checks pass. PR is ready for review. …
--- END DISPATCH CHAT REACTION ---
A reaction, not a new message — reply only if it calls for one (dispatch_chat_post, replyTo: "<id>").

The latest post gets ~100 chars of quote ("your latest question"); older posts up to 300 plus how far back they are. Quoted text passes through escapeEnvelopeMarkers.

Implementation

• DB: 0051_agent-chat-reactions — one row per (message, author, emoji), FK cascade to the message, delivered for user reactions, partial index for the startup recovery sweep. (0050 is the latest migration applied in production.)
• Server: ChatService.addReaction/removeReaction (author-aware), shared injectDetached settlement chain for messages and reactions, POST/DELETE /agents/:id/chat/messages/:messageId/reactions, reactions aggregated onto feed chat rows and republished via chat.entry.
• Web: new chat-reactions.tsx (picker + chip row); framer-motion pop-in/out with sibling slide, row collapses only after the last chip's exit; picker grows open with staggered emoji; reduced-motion respected; picker greys out (not hidden) when the agent can't receive; optimistic toggles that never clobber a newer stream entry.
• Docs: docs/03-api-spec.md Chat section.

Review

• architecture-review (#1341): 1 item (split reaction UI out of chat-entries.tsx) — fixed and verified.
• frontend-ux-review (#1342): 4 items (disabled vs hidden picker, error attribution + in-flight chips, last-chip exit animation, touch affordance) — fixed and verified. The hover/touch × was later removed at the user's request (Slack-style tap-to-remove).

Test plan

• pnpm run check
• pnpm run finalize:web
• pnpm run test (server + web + extension)
• pnpm run test:e2e (198 passed, 12 skipped)
• Playwright on a dev stack: add/remove via picker and chips, agent reactions via MCP with live updates, animation frames sampled, 400px layout

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