Add Slackbot transport support

[victor-develop]

This pull request adds full documentation and configuration support for Slack as a first-class messaging transport in the project, alongside Telegram and Matrix. It updates the README, installation docs, configuration examples, and internationalization files to describe Slack setup, permissions, and usage. No core code changes are included, but all user-facing documentation and config schemas now reflect Slack as a supported option.

Key changes:

Documentation and Setup Instructions:

• Expanded the README.md to include Slack as a supported transport everywhere Telegram and Matrix are mentioned, with detailed setup instructions, required token scopes, event subscriptions, and behavior summary. [1] [2] [3] [4] [5] [6] [7]
• Added a comprehensive Slack setup section to docs/installation.md, including step-by-step app creation, permissions, event subscriptions, allowlist configuration, and example config. [1] [2]

Configuration and Schema:

• Updated config.example.json to show a Slack config block with bot_token, app_token, allowed_channels, and allowed_users.
• Added a SlackConfig model in ductor_bot/config.py and included it in the main AgentConfig schema, allowing Slack configuration to be validated and loaded like other transports. [1] [2]

Module and Capability Descriptions:

• Updated docs/modules/messenger.md to reference Slack as a supported transport, document the SlackBot protocol implementation, and describe Slack's capabilities and factory registration. [1] [2] [3] [4]

Internationalization:

• Added Slack-specific descriptions and footers to the English, German, and Spanish chat translation files. [1] [2] [3] [4] [5] [6]