feat: add Mix tasks for CLI workflow management

[kasvith]

Summary

• Add 5 Mix tasks for managing Durable workflows from the command line: durable.status, durable.list, durable.run, durable.cancel, and durable.cleanup
• Include shared Durable.Mix.Helpers module for table formatting, ID truncation, datetime/duration formatting, and number formatting
• Add tests for all tasks and update README with usage examples

Mix Tasks

Task	Description
mix durable.status	Show queue status and workflow execution summary
mix durable.list	List executions with filters (status, workflow, queue, date range, JSON output)
mix durable.run	Start a workflow with optional JSON input, queue, and priority
mix durable.cancel	Cancel a running/pending/waiting execution with optional reason
mix durable.cleanup	Batch-delete old executions with dry-run support and duration parsing

All tasks support --name for multi-tenancy (selecting a Durable instance).

Test plan

• mix test test/mix/ passes
• mix credo --strict passes
• mix format --check-formatted passes
• Manual smoke test of mix durable.status and mix durable.list against a running app