discord: idiomatic Effect refactor with conversation service, durable ledger, and split sandbox architecture

Refactor the Discord bot to idiomatic Effect TypeScript:

- Branded types (ThreadId, ChannelId, etc.) and Schema.Class for all data
- Split SandboxManager into SandboxProvisioner (stateless lifecycle) + ThreadAgentPool (per-thread orchestration)
- Pure Conversation service with port interfaces (Inbox/Outbox/History/Threads)
- ConversationLedger for message dedup, at-least-once delivery, and replay on restart
- Per-thread serialized execution via ActorMap with idle timeouts
- Discord slash commands (/status, /reset) and in-thread commands (!status, !reset)
- Catch-up on missed messages at startup via offset tracking
- Typed errors (Schema.TaggedError) with retriable/non-retriable classification
- Local CLI (conversation:cli) and automation CLI (conversation:ctl)
- Test coverage for conversation service, ledger, session store, and actors