Feature: Clean API for programmatically starting Shoryuken

[mensfeld]

Summary

Provide a clean, documented API for launching Shoryuken programmatically within Ruby code.

Use Cases

1. Dynamic configuration (build YAML at runtime)
2. Loading environment variables from external sources before startup
3. Adding default middleware before workers are loaded
4. Integration testing with real SQS
5. Embedded Shoryuken in other applications

Current Workaround

Users must call load on the bin script or use undocumented Shoryuken::Runner.instance.run.

Proposed Solution

Document and stabilize the programmatic API:

Shoryuken.boot do |config|
  config.queues = ['my_queue']
  config.concurrency = 10
  # ...
end

Shoryuken.start  # Blocking
Shoryuken.stop   # Graceful shutdown

Original Issue

Originally requested in #680, closed by stale bot. Community workarounds exist in comments.