feat: enable scratchpad tooling frontend and instruction update

- Remove !isScratchpad gate on tooling/approval pills in ChatPane
- Update scratchpad agent guidance to permit tool/file usage
- Add EryxAppService scratchpad tooling integration tests
- Update ARCHITECTURE.md to reflect scratchpad tooling support

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

src/renderer/components/ChatPane.tsx

@@ -369,7 +369,7 @@ export function ChatPane({
           )}
 
           {/* Session config pills — tool & approval controls */}
-          {!isScratchpad && (hasConfigurableTools || hasToolCallApproval) && (
+          {(hasConfigurableTools || hasToolCallApproval) && (
             <div className="mb-2 flex items-center gap-2">
               {hasConfigurableTools && onUpdateSessionTooling && (
                 <InlineToolsPill