Spike: investigate add/remove-node interaction

[lornakelly]

Description

Investigate how authors will add and remove nodes on the diagram for Milestone 3. The ADR explicitly rules out free-form drag-and-drop canvas editing and calls for a simpler, constrained interaction (e.g. an explicit "add node" button) with the exact design to be confirmed. This spike surveys the options, weighs them against our constraints, and produces a recommendation to align on with the team before any implementation tickets are written.

Comment on ticket with investigation and options and discuss with team

Motivation

Milestone 3 is the most complex phase and the interaction model is the biggest open question in it. Committing to an approach without exploring alternatives risks building disproportionate layout/connection complexity for little value.

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.