feat: directional edges and provider icons on agent nodes

- Add ArrowClosed markers to all graph edges for clear directional flow
- Replace generic Bot icon with AI provider logos (OpenAI, Anthropic,
  Google) on agent nodes, inferred from the agent's model id
- Show model display name as subtitle on agent nodes (e.g. 'GPT-5.4',
  'Claude Opus 4.5')
- Thread availableModels catalog from PatternEditor through canvas to
  resolve friendly model names; falls back to raw model id
- Add 3 new tests for arrow markers, provider icons, and model labels

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/components/PatternEditor.tsx

@@ -318,6 +318,7 @@ export function PatternEditor({
           <div className="min-h-[300px] flex-1 px-5 py-3">
             <PatternGraphCanvas
               pattern={pattern}
+              availableModels={availableModels}
               onGraphChange={emitGraphChange}
               onNodeSelect={setSelectedNodeId}
               selectedNodeId={selectedNodeId}