feat: add slack integration

[declanjackson]

Add slack integration

Summary

• Add Slack integration that lets users run Stirrup agents
  from Slack via @mentions with support for named agent
  routing (agent:<name>), model override (model:<name>),
  file attachments, and output file uploads back to the thread
• Includes a zero-config entry point (python -m stirrup.integrations.slack), configurable concurrency,
  skills directory support, and a minimal Python
  logging-based logger for terminal monitoring
• Add documentation guide, README, example script, and
  Dockerfile for the integration