fix(web): align the context separator with ToolRow and name the caption

Review follow-ups on the context provenance row:

- the producer name now follows ToolRow's aria-hidden separator dot instead
  of a literal middot inside the text, so the two disclosure rows expose one
  accessible-name shape;
- the English steering caption becomes the noun `Interjection`, parallel to
  the Chinese 插话 and to the label this bubble carried before;
- the history fold gets its own provenance assertion, so the mapping it
  declares separately from TranscriptAdapter is pinned on both sides;
- the superseded-in-part bullet on the no-interjection-chrome note now
  enumerates the third Decision bullet too, whose no-badge and no-IconActions
  clauses had both lapsed.

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

@@ -165,7 +165,7 @@ describe('MessageItem arms', () => {
       } as never}
       />,
     )
-    const disclosure = ctxView.getByRole('button', { name: /^上下文注入\s*· fixture$/ })
+    const disclosure = ctxView.getByRole('button', { name: /^上下文注入\s*fixture$/ })
     expect(disclosure.getAttribute('aria-expanded')).toBe('false')
     expect(ctxView.container.querySelector('[data-context-injection-body]')).toBeNull()
     expect(ctxView.container.querySelector('svg')).not.toBeNull()
@@ -208,7 +208,7 @@ describe('MessageItem arms', () => {
       } as never}
       />,
     )
-    expect(view.getByRole('button', { name: /^跨会话召回\s*· 重构 loader$/ })).toBeTruthy()
+    expect(view.getByRole('button', { name: /^跨会话召回\s*重构 loader$/ })).toBeTruthy()
     expect(view.queryByText('上下文注入')).toBeNull()
   })
 
@@ -223,10 +223,10 @@ describe('MessageItem arms', () => {
       } as never}
       />,
     )
-    const disclosure = view.getByRole('button', { name: /^上下文注入\s*· AGENTS\.md$/ })
+    const disclosure = view.getByRole('button', { name: /^上下文注入\s*AGENTS\.md$/ })
     fireEvent.click(disclosure)
     expect(disclosure.getAttribute('aria-expanded')).toBe('true')
-    expect(view.container.querySelector('[data-context-source]')?.textContent).toBe('· AGENTS.md')
+    expect(view.container.querySelector('[data-context-source]')?.textContent).toBe('AGENTS.md')
   })
 
   it('a context source that names no producer shows the role alone', () => {