feat(web): name context sources and mark steering in the transcript

Every logged non-user user/message collapsed into one identical
「上下文注入」 row, and mid-turn steering rendered in exactly the bubble a
turn-opening prompt uses, so the transcript could not say what had been
added or which message interrupted a running turn.

TranscriptAdapter and the history fold now attach a provenance view to
each context node, computed by contextProvenance() from the durable
source alone: a role (inject, or recall for a cross-session snapshot)
and a producer name read out of the log — instruction paths for
workspace-instructions, session titles for session-reference, the plugin
id for a plugin source, and the bare kind for any other. No client-side
table of producer names, so a renamed or newly mounted producer stays
identifiable without a client release and a foreign log projects like a
live one. ContextInjectionRow titles itself from the role and shows the
name beside it; MessageItem captions durable and pending steering
bubbles.

The caption reverses one clause of the no-interjection-chrome decision,
which removed it because the composer could not steer; composer steering
shipped afterwards without amending that note, so this change supplies
the product decision its reintroduction clause required and corrects the
stale facts left in it.

Fixes #1291

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

@@ -130,7 +130,7 @@ describe('MessageItem arms', () => {
     fireEvent.click(screen.getByRole('button', { name: '复制' }))
   })
 
-  it('consumed steering renders copy and branch actions without a badge', () => {
+  it('consumed steering is captioned as an interjection and keeps copy and branch actions', () => {
     const writeText = vi.fn().mockResolvedValue(undefined)
     Object.defineProperty(navigator, 'clipboard', {
       configurable: true,
@@ -145,7 +145,7 @@ describe('MessageItem arms', () => {
       onFork={fork}
       />,
     )
-    expect(view.queryByText('插话')).toBeNull()
+    expect(view.getByText('插话')).toBeTruthy()
     expect(view.getByText('steer!')).toBeTruthy()
     expect(view.getByText(/附加内容块/)).toBeTruthy()
     fireEvent.click(view.getByRole('button', { name: '复制' }))
@@ -161,10 +161,11 @@ describe('MessageItem arms', () => {
         seq: 3,
         content: [{ type: 'text', text: 'x\n"y":,[{}]' }],
         source: { kind: 'plugin', plugin: 'fixture', empty: {}, list: [] },
+        provenance: { role: 'inject', label: 'fixture' },
       } as never}
       />,
     )
-    const disclosure = ctxView.getByRole('button', { name: '上下文注入' })
+    const disclosure = ctxView.getByRole('button', { name: /^上下文注入\s*· fixture$/ })
     expect(disclosure.getAttribute('aria-expanded')).toBe('false')
     expect(ctxView.container.querySelector('[data-context-injection-body]')).toBeNull()
     expect(ctxView.container.querySelector('svg')).not.toBeNull()
@@ -187,6 +188,7 @@ describe('MessageItem arms', () => {
         seq: 3,
         content: [{ type: 'text', text: 'x'.repeat(21_000) }],
         source: null,
+        provenance: { role: 'inject', label: null },
       } as never}
       />,
     )
@@ -195,6 +197,50 @@ describe('MessageItem arms', () => {
       .toMatch(/… 已截断，共 \d+ 字符$/)
   })
 
+  it('a recalled session titles its row by role and names the sessions it read', () => {
+    const view = render(
+      <MessageItem t={t} node={{
+        kind: 'context',
+        seq: 3,
+        content: [{ type: 'text', text: 'snapshot' }],
+        source: { kind: 'session-reference', version: 1, references: [{ label: '重构 loader' }] },
+        provenance: { role: 'recall', label: '重构 loader' },
+      } as never}
+      />,
+    )
+    expect(view.getByRole('button', { name: /^跨会话召回\s*· 重构 loader$/ })).toBeTruthy()
+    expect(view.queryByText('上下文注入')).toBeNull()
+  })
+
+  it('keeps the producer name visible while the context body is expanded', () => {
+    const view = render(
+      <MessageItem t={t} node={{
+        kind: 'context',
+        seq: 3,
+        content: [{ type: 'text', text: 'instructions' }],
+        source: { kind: 'workspace-instructions', changes: [{ path: 'AGENTS.md' }] },
+        provenance: { role: 'inject', label: 'AGENTS.md' },
+      } as never}
+      />,
+    )
+    const disclosure = view.getByRole('button', { name: /^上下文注入\s*· AGENTS\.md$/ })
+    fireEvent.click(disclosure)
+    expect(disclosure.getAttribute('aria-expanded')).toBe('true')
+    expect(view.container.querySelector('[data-context-source]')?.textContent).toBe('· AGENTS.md')
+  })
+
+  it('a context source that names no producer shows the role alone', () => {
+    const view = render(
+      <MessageItem t={t} node={{
+        kind: 'context', seq: 3, content: [{ type: 'text', text: 'x' }], source: null,
+        provenance: { role: 'inject', label: null },
+      } as never}
+      />,
+    )
+    expect(view.getByRole('button', { name: '上下文注入' })).toBeTruthy()
+    expect(view.container.querySelector('[data-context-source]')).toBeNull()
+  })
+
   it('unknown nodes retain the generic JSON row', () => {
     const unknownView = render(
       <MessageItem t={t} node={{ kind: 'unknown', seq: 4, type: 'surface/next', data: { x: 1 } } as never} />,