Add concurrent steps support to workflow definitions

[nhorton]

Summary

This PR adds support for concurrent (parallel) step execution in workflow definitions, allowing workflows to specify which steps can run in parallel while maintaining full backward compatibility with existing sequential workflows.

Key Changes

Parser & Data Model

• Added WorkflowStepEntry dataclass to represent workflow step entries that can be either sequential (single step) or concurrent (multiple steps)
• Modified Workflow class to use step_entries internally while maintaining a backward-compatible steps property that flattens all step IDs
• Added helper methods to Workflow: get_step_entry_for_step() and get_entry_index_for_step()

Job Definition API

• Added get_step_entry_position_in_workflow() method to get entry-based position of a step (useful for concurrent groups where multiple steps share the same position)
• Added get_concurrent_step_info() method to get a step's position within its concurrent group (returns tuple of position and group size)

Schema & Validation

• Updated JSON schema to support both string step IDs and arrays of step IDs in workflow definitions
• Single-item arrays indicate a step with multiple parallel instances (e.g., [fetch_campaign_data] for per-campaign execution)
• Multi-item arrays indicate steps that execute concurrently (e.g., [task_a, task_b, task_c])

Code Generation

• Enhanced meta-skill template to render concurrent steps as "Background Task 1/2/3" with clear instructions
• Updated _build_meta_skill_context() to include step_entries with detailed concurrency information
• Concurrent steps are rendered with task numbers and descriptions for clarity

Documentation & Examples

• Updated architecture documentation with concurrent step patterns and examples
• Added comprehensive test fixture concurrent_steps_job demonstrating a workflow with parallel research phase
• Updated CHANGELOG with feature description

Backward Compatibility

• Existing workflows with simple step arrays continue to work unchanged
• The workflow.steps property provides flattened list of all step IDs for backward compatibility
• All existing code using step lists continues to function without modification

Testing

• Added comprehensive test suite covering concurrent step parsing, validation, and code generation
• Tests verify both the internal step_entries structure and backward-compatible steps property
• Tests validate meta-skill generation with proper concurrent step rendering

https://claude.ai/code/session_011qMp5U4AFQLMbJQvCE8Ck4