Add mention support and user restrictions for commands

[avrabe]

Summary

• Add bot_name config to customize mention format (e.g., @temper[bot])
• Add allowed_command_users to restrict who can trigger commands
• Support both /command and @botname command syntax

Changes

• config.yml.example: Add new config options
• src/schema.js: Add validation for new config fields
• src/app.js: Parse mentions and check allowed users

Usage

After merging, users can trigger commands via:

• /configure-repo (existing)
• @temper[bot] configure-repo (new)

Configure allowed users in config.yml:

bot_name: temper
allowed_command_users:
  - avrabe
  - other-admin