fix: agent node deletion, inspector layout, and header overlap

- Enable Delete-key removal of agent nodes on the canvas; system nodes
  remain non-deletable. Removals route through the authoritative
  removeAgent path which prevents deleting the last agent.
- Stack Model and Reasoning selects vertically in the inspector panel
  so they fit within the 320px-wide column without truncation.
- Add right padding (pr-36 / 144px) to the editor header so the Save
  and Delete buttons clear the Windows title-bar overlay controls.

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

src/renderer/components/pattern-graph/PatternGraphInspector.tsx

@@ -184,7 +184,7 @@ function AgentNodeInspector({
         value={agent.name}
       />
 
-      <div className="grid gap-3 sm:grid-cols-2">
+      <div className="space-y-3">
         <ModelSelect
           models={availableModels}
           onChange={(value) => {