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/PatternEditor.tsx

@@ -231,7 +231,7 @@ export function PatternEditor({
   return (
     <div className="flex h-full flex-col">
       {/* Header */}
-      <div className="drag-region flex items-center justify-between border-b border-[var(--color-border)] px-5 pb-3 pt-3">
+      <div className="drag-region flex items-center justify-between border-b border-[var(--color-border)] pb-3 pl-5 pr-36 pt-3">
         <div className="flex items-center gap-3">
           <button
             className="no-drag flex size-8 items-center justify-center rounded-lg text-zinc-400 transition hover:bg-zinc-800 hover:text-zinc-200"
@@ -320,6 +320,7 @@ export function PatternEditor({
               pattern={pattern}
               availableModels={availableModels}
               onGraphChange={emitGraphChange}
+              onAgentRemove={removeAgent}
               onNodeSelect={setSelectedNodeId}
               selectedNodeId={selectedNodeId}
             />