`discord` provider (`list` + `comments`)

[abdotop]

Depends on: Issue 1, Issue 2

Implement the discord Provider:

• list(scope) — query discord/channel (or thread listing) scoped by
  scope.discordChannelId, match thread names prefixed with {KEY}, map
  to WorkItem[]
• comments(item) — query discord/message + discord/message-bot +
  discord/bot-message select(.channel_id == item.externalId) (see §5)

Acceptance criteria

• list() finds real project threads for the pilot project's Discord
  channel and extracts the correct key
• comments() returns the full message history for one real thread,
  scoped server-side (never pulling the whole discord/message
  collection client-side — see §7's store-load risk)