feat(ai-studio): node types show their prototype accent colors

[jimmeryn]

What changed

Every AI Studio node type now declares an accent on its definition, so the canvas and the palette show the prototype colors:

Node type	Accent
Trigger	orange
AI Agent	violet-gradient
Decision	green
Human task	violet
Visualize	green

• A spec over the palette definitions fails when a node type has no accent or a different one, so a node type added later cannot ship uncolored.
• A render test checks that the Human task template passes the accent to its icon.

Decisions

• The color comes from the definition by node type and is never written into node data, templates or saved diagrams. Workflows saved earlier get the colors too.
• AI Agent sets violet-gradient explicitly. It renders the generic node template, which has no gradient default, and the Properties header reads only the definition.
• Visualize is green for now. The prototype has no Visualize node; its closest one (Display value) uses the logic green.
• No changeset: AI Studio is private and consumes the SDK from the workspace.

Open questions

• Visualize color still needs design confirmation. A different answer is a one-line change.
• The Properties header color can be checked only after the Properties header work (feat/properties-header-node) merges.
• The Review node, still on its own branch, will need an accent. Whichever PR merges second adds it; the spec will flag it.

Verified

• AI Studio suite, typecheck, lint.
• Manually: canvas and palette, light and dark, Customer Support Triage and Refund Review.