Move message splitting from individual channels (Discord) to the Manager
layer via per-channel worker goroutines. Each channel now declares its
max message length through BaseChannelOption/MessageLengthProvider, and
the Manager automatically splits oversized outbound messages before
dispatch. This prevents one slow channel from blocking all others.
- Add WithMaxMessageLength option and MessageLengthProvider interface
- Set platform-specific limits (Discord 2000, Telegram 4096, Slack 40000, etc.)
- Convert SplitMessage to rune-aware counting for correct Unicode handling
- Replace single dispatcher goroutine with per-channel buffered worker queues
- Remove Discord's internal SplitMessage call (now handled centrally)
Add bus.Peer struct and explicit Peer/MessageID fields to InboundMessage,
replacing the implicit peer_kind/peer_id/message_id metadata convention.
- Add Peer{Kind, ID} type to pkg/bus/types.go
- Extend InboundMessage with Peer and MessageID fields
- Change BaseChannel.HandleMessage signature to accept peer and messageID
- Adapt all 12 channel implementations to pass structured peer/messageID
- Simplify agent extractPeer() to read msg.Peer directly
- extractParentPeer unchanged (parent_peer still via metadata)
Hoshina