When sending Discord messages via cron jobs or the message tool,
usernames like "john.doe" were incorrectly treated as channel names,
causing silent delivery failures.
This fix adds a resolveDiscordTarget() function that:
- Queries Discord directory to resolve usernames to user IDs
- Falls back to standard parsing for known formats
- Enables sending DMs by username without requiring explicit user:ID format
Changes:
- Added resolveDiscordTarget() in targets.ts with directory lookup
- Added parseAndResolveRecipient() in send.shared.ts
- Updated all outbound send functions to use username resolution
Fixes#2627
