feat: Host-agnostic editable text + change events #224

Description

Depends on: #208
Editor decided (#208): Monaco.

Integrate chosen code editor in new package and give the editor an editable text surface + a change-event contract, host-agnostic (doesn't assume where it runs or who embeds it e.g. a plain web page / Storybook, the Quarkus Dev UI, a VS Code or IntelliJ extension etc).

Already exists: the provider re-parses and redraws whenever the content prop changes (parseWorkflow(props.content) in a useMemo,DiagramEditorContextProvider.tsx). The gap: there is no editable text field and noonChange emitted out — content is a one-way input prop. This ticket adds the editable surface and the change contract so edits flow, while reusing the existing prop→re-parse→redraw plumbing.

Source of truth: The editor is a controlled text component: it surfaces the current text as content and reports edits via onChange; the parsed model and the diagram are always a derived view of that text.

Motivation

Without an editable surface + onChange contract, we can't support any real editing workflow

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.