Add dm_to_another tool for sending DMs to other users

[JasonWang1124]

Summary

Add a new MCP tool dm_to_another that allows sending direct messages to
other users, complementing the existing dm_self tool.

Fixes #461

Changes

• Add DMToAnotherArgs struct with user_id, username, message, and
  attachments fields
• Register dm_to_another tool in getPostTools()
• Implement toolDMToAnother method using CreateDirectChannel API
• Support finding target user by either user_id or username

Implementation Details

The implementation follows the same pattern as dm_self:

1. Find target user by user_id (using GetUser) or username (using
   GetUserByUsername)
2. Create or get existing DM channel using
   CreateDirectChannel(currentUser.Id, targetUser.Id)
3. Create the post in the DM channel with appropriate props
   (from_webhook, ai_generated_by)

Example Usage

{"username": "john.doe", "message": "Hello!"}

or

{"user_id": "abc123xyz456", "message": "Meeting reminder"}

Testing

- Tested sending DM to another user by username ✅
- Tested sending DM to another user by user_id ✅
- Verified bot label is displayed (consistent with dm_self) ✅

Add `dm_to_another` MCP tool to send direct messages to other users