discord: simplify ConversationLedger to offsets + in-memory dedup

Replace the full inbox state machine (admit/start/setTarget/setPrompt/
setResponse/complete/retry/prune/replayPending) with three methods:
dedup (bounded in-memory Set), getOffset, and setOffset.

Discord is already the durable inbox — on startup we just resume from
persisted offsets per source. Within a session, in-memory dedup prevents
double-processing. This removes ~565 lines of inbox lifecycle code,
the conversation_inbox SQLite table, MessageState, ReliabilityError,
and the prune schedule.