/upcoming and /whos-in

Two of the four commands. Both read-only, both ephemeral, neither edits anything.

- [ ] `/upcoming` — ephemeral agenda across every campaign the caller is on: date, session number, confirmed/in-jeopardy, their own intent. Replaces the idea of a pinned board: always current, no channel noise.
- [ ] `/whos-in` — authoritative roster state for the next (or a chosen) session, from D1, for when a post has gone stale. Includes flake memory where it says something.
- [ ] Register both in `src/discord/commands.ts` with the option shapes; `npm run commands:register`.

Start with the command definitions and option shapes in src/discord/commands.ts, then inspect the existing campaign, session, and D1 data paths used by Discord commands. Run npm run commands:register after adding both registrations. Done means /upcoming and /whos-in provide the listed read-only, ephemeral agenda and roster information for the relevant campaigns and sessions.

索引モデルが issue の本文から書いたものです。