Seed a start node when a new workflow is created

Problem/Motivation

Every valid workflow needs exactly one start node, but a brand-new workflow opens with an empty canvas, so the author must manually add the start node before doing anything else. That first step is pure boilerplate.

In #3607005 the add-node control was changed to offer and allow the start type only while no start node exists yet, and validation now refuses a second start. That correctly closed the duplicate-start bug, but it left creating the first start as a manual step.

Proposed resolution

• Seed a single start node automatically when a new workflow is created, in both authoring paths: the orchestra_cm form new-model path, and the BPMN.io modeler new-model creation, keeping the two in parity.
• Once a start always exists, simplify the add-node control back to an unconditional exclusion of the start type.
• Update the authoring tests (OrchestraCmAuthoringTest, OrchestraCmFeatureAuthoringTest) to expect the pre-seeded start instead of adding it.

Remaining tasks

• Implement start-node seeding on new-workflow create.
• Make the orchestra_cm form and the BPMN.io modeler behave identically.
• Simplify the add-node control and its validation.
• Update and run the authoring tests.

User interface changes

A newly created workflow opens with a start node already present.