Adds two new internal hook events that fire after media/link processing:
- message:transcribed: fires when audio has been transcribed, providing
the transcript text alongside the original body and media metadata.
Useful for logging, analytics, or routing based on spoken content.
- message:preprocessed: fires for every message after all media + link
understanding completes. Gives hooks access to the fully enriched body
(transcripts, image descriptions, link summaries) before the agent sees it.
Both hooks are added in get-reply.ts, after applyMediaUnderstanding and
applyLinkUnderstanding. message:received and message:sent are already
in upstream (f07bb8e8) and are not duplicated here.
Typed contexts (MessageTranscribedHookContext, MessagePreprocessedHookContext)
and type guards (isMessageTranscribedEvent, isMessagePreprocessedEvent) added
to internal-hooks.ts alongside the existing received/sent types.
Test coverage in src/hooks/message-hooks.test.ts.