Epic: Milestone 3 - Build & wire the workflow

[lornakelly]

Description

Turn the diagram from editable (Milestone 2) into constructable: build and reshape the whole workflow visually - add and remove nodes, and wire the flow between them. Per the ADR, text stays the source of truth - every structural change is written back into the text, which remains fully round-trippable.

Not in scope: free-form drag-and-drop canvas editing

Motivation

Completes the read-only-viewer → full-visual-authoring arc. Authors can compose a workflow end-to-end on the canvas without touching nested text by hand, making the spec approachable to build with, not just to read.

Proposed Implementation

No response

Definition of Done

• Implementation: Fully implemented according to the Serverless Workflow spec.
• Unit Tests: Comprehensive unit tests are included and passing.
• Integration Tests: Verified within the monorepo and target environments (Web/VS Code).
• Documentation: Updated README.md, ADRs, or official docs.
• Performance: No significant regression in editor responsiveness.
• Accessibility: UI changes comply with accessibility standards.