[effect-bot]

A 👍 reaction (any skin tone) on one of the chat bot's Slack or Discord messages now counts as approval. It works when the reaction comes from an admitted user, in a thread that has a session. The session gets it as a message from that user, "👍 (reacted with a thumbs up to your message: approved)". That message counts as a mention, so it skips the reply gate and starts or steers a turn. This lets someone answer a yes/no question or an approval request with a thumbs up.

Authorization stays as it is. A reaction never starts a session. Other emoji, reactions on someone else's message, and reactions from outsiders, guests or bots are ignored, not kept as context. Reactions from other guilds and in threads without a session are ignored too.

Discord

• DiscordIngress.handleReaction handles MESSAGE_REACTION_ADD. The gateway now asks for GUILD_MESSAGE_REACTIONS, which is not privileged, and routes the event through the same before-sequence hook as messages. Approval ids go into SeenMessages, so a RESUME replay is dropped.
• Admission uses the reaction's member.roles, through a toAuthor helper that is now shared with toChatMessage.
• Discord sends a Unicode emoji as { id: null, name: "👍" }, although dfx types it as a custom emoji. Matching is by name, and only for emoji with no id.
• postReply: an approval's id (reaction/{message}/{user}) isn't a Discord snowflake, and Discord would reject a message_reference to it. A turn started by an approval therefore mentions the approver instead of replying to the message, the same way the thread-starter case already works.

Slack

• The webhook decodes reaction_added. It accepts +1 or thumbsup, with or without ::skin-tone-N, on a message whose item_user is the bot.
• The event names the message but not its thread, so ingress calls conversations.replies with ts = the reacted message. Per the API docs, ts may be "either a thread's parent message or a message in the thread". Results are ordered earliest first, and every threaded message carries the root's ts as thread_ts, so messages[0].thread_ts is the root. An unthreaded message has no thread_ts and is ignored. A denied error, such as thread_not_found for a deleted message, is logged and the reaction ignored. Other failures answer 503 so Slack retries. The call needs no new scopes: channels:history, groups:history and im:history are already granted.
• The admission path (session cache, then users.info, then remember) is factored into admit and shared with the message path.

Deployment

• Slack: both apps need the reactions:read scope and a reaction_added bot event subscription, then a reinstall.
• Discord: nothing to toggle in the portal; the new intent is picked up when the gateway next IDENTIFYs.

docs/chat.md covers both.

Validation

• I added one regression test per platform in test/Chat/{Slack,Discord}/Ingress.test.ts. Each covers the approval cases and all the dropped cases. Both fail on main and pass here.
• vp check passes. All Chat tests pass (95/95).
• The full backend suite had 12 timeouts in Session and PullRequestReview tests on a heavily loaded runner (load average above 20). The Session tests pass with a 30 s timeout.

Closes EFF-3163