Summary

Fixes #2627

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.

Problem

The Discord outbound adapter had no `resolveTarget` hook. The `parseDiscordTarget()` function fell back to treating unrecognized strings as channel names:
```typescript
return buildMessagingTarget("channel", trimmed, trimmed);
```

So `john.doe` was passed as a channel name lookup, which fails because no channel named `john.doe` exists.

Solution

This fix adds a `resolveDiscordTarget()` function that:

• Queries Discord directory to resolve usernames to user IDs using `listDiscordDirectoryPeersLive()`
• Falls back to standard parsing for known formats (user:, channel:, @mentions, numeric IDs)
• Enables sending DMs by username without requiring explicit `user:ID` format

Changes

• src/discord/targets.ts: Added `resolveDiscordTarget()` function with directory lookup
• src/discord/send.shared.ts: Added `parseAndResolveRecipient()` wrapper function
• src/discord/send.outbound.ts: Updated `sendMessageDiscord()`, `sendStickerDiscord()`, and `sendPollDiscord()` to use username resolution

Testing

After this fix, cron jobs can target usernames:
```json
{
"to": "john.doe",
"channel": "discord"
}
```

The username will be resolved to a Discord user ID, and the message will be delivered as a DM.

Note: This is a first step. For username resolution to work, Discord directory integration must be configured and bot must have access to guild member lists.