Add example: Slack bot for CTI queries

[rolandpg]

Summary

Create an example Slack bot that uses ZettelForge for CTI memory.

Context

A Slack bot example shows how to integrate ZettelForge into a real-world workflow. Analysts can query threat intel from Slack.

• File to create: examples/slack_bot.py
• Framework: slack_bolt

Acceptance Criteria

• Self-contained script with clear setup instructions in a docstring
• Uses slack_bolt for the bot framework
• Implements at least 3 slash commands:
  • /remember <text> — store CTI content
  • /recall <query> — search memories
  • /entity <type> <value> — fast entity lookup (e.g., /entity actor apt28)
• Bot responses include extracted entities and confidence scores
• Environment variables for config: SLACK_BOT_TOKEN, SLACK_APP_TOKEN
• Has a requirements.txt or pip install line in the docstring
• Includes a "Setup" section explaining: create Slack app, enable socket mode, add scopes, install to workspace
• Code runs without errors when Slack tokens are not set (graceful error message)