fix(web): drop the Close button from the read-only preset view

A shipped preset has nothing to commit or abandon, and the back link above
already leaves the screen; a lone Close button below the composition was a
second way out of the same place. The action row now renders only when the
draft is writable, where Cancel and Save are both real choices.

The composition label named `cordis.yml`; the file a preset actually holds
is `agent.cordis.yml`.

packages/client/ui-agent-preset/tests/section.spec.tsx

@@ -239,7 +239,10 @@ describe('the composition editor', () => {
     expect(screen.getByLabelText(en.composition)).toHaveProperty('readOnly', true)
     expect(screen.getByText(en.readOnlyNotice)).toBeTruthy()
     expect(screen.queryByText(en.save)).toBeNull()
-    expect(screen.getByText(en.close)).toBeTruthy()
+    // Nothing to commit or abandon, and the back link above already leaves —
+    // a lone Close button would be a second way out of the same screen.
+    expect(screen.queryByText(en.cancel)).toBeNull()
+    expect(screen.getByRole('button', { name: `← ${en.backToList}` })).toBeTruthy()
   })
 
   it('titles the panel by what it is doing', () => {