Replace 'thinking' message with a reaction on the user's message #28

The current pattern in app.js — post a "thinking" message, call the LLM, delete the message, post the reply — works but feels clunky and creates message churn. Delete failures need to be logged around.

Proposal

When a message comes in that will be routed to the LLM:

1. reactions.add :brain: (or :hourglass_flowing_sand:) to the user's message
2. Call handleMessage
3. reactions.remove the reaction
4. Post the reply

Cleaner Slack UX, no posted-then-deleted message, no race on chat.delete failures.

Touch points

• postThinking / clearThinking helpers in app.js → replace with addThinkingReaction / removeThinkingReaction.
• Both message handlers (general + direct mention).
• Update tests if any cover the thinking UX.

Caveat

DMs require the bot to have reactions:write scope. Check manifest.yaml and re-sync if needed.