cleanup(web): drop the unimplemented user-message edit control

The user bubble's IconActions row carried an edit button with no click
handler, no client mutation, and no host operation behind it, advertising
an affordance the product cannot honor. The row now renders clock / copy /
branch only for both user and assistant chrome, and MessageIconActions
loses the `edit` prop with the button.

The package README records the missing capability under Known Limitations,
the web aria goldens pin the row without the control, and the Agent Note
states what must exist before it comes back.

packages/client/ui-conversation/tests/chat-branch-tails.spec.tsx

@@ -27,7 +27,7 @@ afterEach(() => {
 const t: MessageItemProps['t'] = makeTranslate(zh, commonZh)
 
 describe('MessageItem arms', () => {
-  it('user bubbles expose clock / copy / branch / edit; copy writes the text', () => {
+  it('user bubbles expose clock / copy / branch and no edit; copy writes the text', () => {
     const writeText = vi.fn().mockResolvedValue(undefined)
     Object.defineProperty(navigator, 'clipboard', {
       configurable: true,
@@ -47,7 +47,7 @@ describe('MessageItem arms', () => {
     expect(screen.getByText('14:24')).toBeTruthy()
     expect(screen.getByRole('button', { name: '复制' })).toBeTruthy()
     expect(screen.getByRole('button', { name: '在新对话中分支' })).toBeTruthy()
-    expect(screen.getByRole('button', { name: '编辑' })).toBeTruthy()
+    expect(screen.queryByRole('button', { name: '编辑' })).toBeNull()
     fireEvent.click(screen.getByRole('button', { name: '复制' }))
     expect(writeText).toHaveBeenCalledWith('hello bubble')
   })