S3 — Slack: @mention to threaded reply

[fmind]

What a stranger gets: @a4h docs hello in a Slack channel answers in the thread.

Acceptance

1. Socket Mode (xoxb- + xapp- tokens), no inbound port; app_mention events acknowledged within 3 seconds.
2. The reply lands in the thread (event.thread_ts ?? event.ts).
3. At-least-once delivery handled: in-memory dedup window; a Slack retry produces exactly one reply.
4. One process per app token — multiple Socket Mode connections are never assumed.
5. Live-verified against the dev workspace (TODO items 3 and 6); fixture tests stay hermetic.

Non-goals

Streaming and edits (S5) · Block Kit richness · anything Marketplace.