Warn on action nodes with no incoming edges

[bender-rodriguez-unit1]

Problem

Action nodes (components) without any incoming edges will never execute — they're dead nodes on the canvas. Currently there's no warning for this, so users can accidentally leave nodes unconnected without realizing they'll never fire.

Current behavior

• Missing required config fields → warning ✅
• Node with no incoming edges → no warning ❌

Expected behavior

If an action/component node has zero incoming edges, show a warning like:

This node has no incoming connections and will never be triggered.

This should apply to TYPE_COMPONENT nodes only. Trigger nodes are entry points so they don't need incoming edges.

Why this matters

When edges use the wrong output channel (e.g. default instead of success), the edge exists in the spec but doesn't actually connect visually or functionally. The downstream node effectively has no valid incoming edges and will never execute. A warning here would immediately surface wiring mistakes.

Example

A canvas with:

nodes:
  - { id: trigger, type: TYPE_TRIGGER, trigger: { name: start } }
  - { id: step-a, type: TYPE_COMPONENT, component: { name: http } }
  - { id: step-b, type: TYPE_COMPONENT, component: { name: http } }  # no edges point here
edges:
  - { sourceId: trigger, targetId: step-a, channel: default }

step-b should show: ⚠️ "No incoming connections — this node will never be triggered."