[Improve] Match Slack emoji reactions on Discord

[roomote-roomote]

Opened on behalf of Matt Rubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Discord.

What changed

Discord reaction behavior is now aligned with Slack:

• Platform adds 👀 on real MESSAGE_CREATE launches; /new interaction launches pin 👀 on the posted acknowledgement instead of using interaction IDs
• Launch payloads store a real discordReactionChannelId / discordReactionMessageId target for later merge/close/cancel reactions
• Slack-style reaction names map to Discord unicode (white_check_mark, x, thumbsdown, thumbsup, etc.)
• Non-Slack agent instructions allow/prefer send_chat_reaction_emoji when policy allows
• PR terminal: ✅ merge / 👎 closed, and remove 👀, only when a valid message target exists
• Cancel button: ❌ on the stored reaction target
• Focused tests cover the above, including interaction launches that never react on interaction snowflakes

Why this change was made

Discord was missing Slack-parity reactions, and an earlier pass incorrectly reacted on interaction IDs (which Discord rejects). That made slash-command tasks never show the advertised reactions.

Impact

Users should see eyes on message-based and /new launches that produce a real acknowledgement, plus terminal/cancel emojis when those stored targets exist.

[roomote-roomote]

1 issue outstanding. See task

• apps/api/src/handlers/discord/index.ts:484 /new persists its interaction ID as communicationSourceEventId, but Discord reactions require a real message ID. The initial eyes reaction still 404s silently, and the new terminal/cancel paths reuse that same ID, so their advertised reactions also never land for slash-command tasks. Persist a valid message-and-channel reaction target for interaction launches, or omit reactions when none exists.
• apps/api/src/handlers/discord/index.ts:464 Snapshot-resumed Discord runs only preserve discordTaskThread; the new discordReactionChannelId and discordReactionMessageId are dropped. Because terminal notifications now react only through those fields, a PR created by a resumed Discord task posts its status but cannot add the terminal emoji or remove eyes. Preserve a valid reaction target during snapshot resume and cover that path.

_{Reviewed b970367}

[roomote-roomote]

Fixed the open review finding.

What changed: /new and other interaction launches no longer try to react with interaction IDs. Real MESSAGE_CREATE still 👀 on the origin message; interaction launches pin 👀 after the acknowledgement posts and store discordReactionChannelId / discordReactionMessageId for merge/close/cancel. Terminal and cancel reactions only run when that real target exists.

Commit: b970367