feat: remove legacy OpenCode runtime

Cut product flows over to Coding/Pi and retain only the migration-owned v1 boundary. Promote supported native optional packages because electron-builder omitted pnpm transitive optional closure from the packaged ASAR.

tests/unit/subagents-page.test.tsx

@@ -4,17 +4,17 @@ import { describe, expect, it } from 'vitest';
 import { Subagents } from '@/pages/Subagents';
 
 describe('Subagents retired route', () => {
-  it('redirects the old course path page to the opencode chat page', async () => {
+  it('redirects the old course path page to the coding chat page', async () => {
     render(
       <MemoryRouter initialEntries={['/subagents']}>
         <Routes>
           <Route path="/subagents" element={<Subagents />} />
-          <Route path="/opencode-chat" element={<div data-testid="opencode-chat-page" />} />
+          <Route path="/chat" element={<div data-testid="coding-chat-page" />} />
         </Routes>
       </MemoryRouter>,
     );
 
-    expect(await screen.findByTestId('opencode-chat-page')).toBeInTheDocument();
+    expect(await screen.findByTestId('coding-chat-page')).toBeInTheDocument();
     expect(screen.queryByTestId('subagents-page')).not.toBeInTheDocument();
   });
 });