CedarCopilot/cedar-mail

Branch: staging

feat(slack): add Send action button for drafted emails

merged#2821CedarCopilot

CedarCopilot wants to merge 3 commits into staging from feat/slack-send-email-action

What Libra verified

Draft threading and attachments survive the send path
Libra ran this behavior against the change and confirmed the check detects when it breaks.
Technical evidence
- a fresh Gmail draft omits its message-self thread ID instead of treating the draft as a reply
- a reply draft retains its existing Gmail thread ID for the outgoing message
- draft attachments remain attached to the parsed draft data used by the send path
Changed code: google.ts.