Phase 10: Define TypingCapable, MessageEditor, PlaceholderRecorder interfaces.
Manager orchestrates outbound typing stop and placeholder editing via preSend.
Migrate Telegram, Discord, Slack, OneBot to register state with Manager instead
of handling locally in Send. Phase 7: Add native WebSocket Pico Protocol channel
as reference implementation of all optional capability interfaces.
Define sentinel error types (ErrNotRunning, ErrRateLimit, ErrTemporary,
ErrSendFailed) so the Manager can classify Send failures and choose the
right retry strategy: permanent errors bail immediately, rate-limit
errors use a fixed 1s delay, and temporary/unknown errors use exponential
backoff (500ms→1s→2s, capped at 8s, up to 3 retries). A per-channel
token-bucket rate limiter (golang.org/x/time/rate) throttles outbound
sends before they hit the platform API.