Python: Add functional workflow API

[moonbox3]

Summary

Feedback that we should add a functional API for defining workflows, in addition to the current graph-based API.

Context

Currently workflows are defined using builder patterns:

workflow = (
    WorkflowBuilder()
    .register_executor(...)
    .add_edge(...)
    .build()
)

A functional API could look like:

workflow = Workflow(
    executors={...},
    edges=[...],
)

Action Items

• Design functional API surface
• Consider relationship with builder pattern (complement vs replace)
• Evaluate if this aligns with "direct Workflow instantiation" feedback in builder ergonomics issue
• Prototype and gather feedback