Adds GGT compatibility flags to bd mail send:
- --priority N: Explicit priority (0-4), overrides --urgent
- --type: Message type (task, scavenge, notification, reply)
- --thread-id: Thread ID for conversation grouping
- --reply-to: Message ID for threading via dependency
This fixes UX inconsistency where gt mail send passed flags that
bd mail send did not support (gt-8j8e).
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
b491f4a9d0