bug(messages): email getThread() is stub, inconsistent adapter semantics

[willgriffin]

Problem

1. Email getThread() returns single message — stub implementation, no real threading
2. Inconsistent connection semantics — Twitter requires connect() before fetch(), Slack doesn't
3. Slack message IDs use undocumented composite encoding — channel:ts format not surfaced in types
4. OAuth 1.0a signing is hand-rolled — fragile, no library backing

Found during

CLAUDE.md audit — packages/messages/src/