feat(web): declare the remaining context forms on every shipped producer

Four values complete the vocabulary, so the opaque body is reached only by
producers that genuinely promise no shape.

`snapshot` — current state a later snapshot supersedes. system-prompt now
exposes `renderContextSections()`, the named contributions
`renderContextSnapshot()` already joins for the model, so the body attributes
each part to the subsystem that produced it instead of re-splitting joined
prose. The runtime snapshot, time-context, and tmux-context declare it.

`notice` — a one-off account of what just happened, declared by tool-tasks,
goal state changes, tool-goal wrap-up, plan-mode switches, and
repeat-tool-guard. Its `summary` rides the COLLAPSED row: these five are the
majority of shipped producers and none of them needs expanding to be read.
The task summary bounds itself because its inputs are unbounded caller text.

`relay` — a message another agent addressed to this one; both subagent
sources declare it and the body names the sender above what it said.

`recall` — material lifted from another session's log. session-reference
needed no new field: its references already record retained and omitted
counts and the truncation flag, which the body shows first, because recalled
context is bounded on the way in.

`ContextFormed` is now discriminated by `form`, so a producer cannot declare
a shape without the facts that shape is presented from — a notice without its
summary, or a snapshot without its sections, fails to compile.

Only the two hook bridges stay opaque, by design: their content is whatever
an external program printed, so no shape can be promised for it. Unknown
kinds and unreadable records land there too.

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

@@ -562,6 +562,106 @@ describe('MessageItem arms', () => {
     expect(fields).toEqual(['plugin', 'form'])
   })
 
+  it('the snapshot form attributes each part to the subsystem that produced it', () => {
+    const view = render(
+      <MessageItem t={t} node={{
+        kind: 'context',
+        seq: 3,
+        content: [{ type: 'text', text: 'Current runtime context.\n\nsandbox\n\nworkspace' }],
+        source: {
+          kind: 'plugin',
+          plugin: '@deepseek-ai/dsh-system-prompt',
+          form: 'snapshot',
+          sections: [{ name: 'sandbox:policy', text: 'workspace-write' }, { name: 'workspace', text: '/repo' }],
+        },
+        provenance: { role: 'inject', label: '@deepseek-ai/dsh-system-prompt' },
+        form: 'snapshot',
+      } as never}
+      />,
+    )
+    fireEvent.click(view.getByRole('button', { name: /^上下文注入\s*@deepseek-ai\/dsh-system-prompt$/ }))
+    const rows = [...view.container.querySelectorAll('[data-context-sections] div')].map(node => node.textContent)
+    expect(rows).toEqual(['sandbox:policyworkspace-write', 'workspace/repo'])
+  })
+
+  it('a notice puts its account on the collapsed row', () => {
+    // The whole point of the form: readable without expanding.
+    const view = render(
+      <MessageItem t={t} node={{
+        kind: 'context',
+        seq: 3,
+        content: [{ type: 'text', text: 'background task bash-1 finished.' }],
+        source: { kind: 'plugin', plugin: 'tool-tasks', form: 'notice', summary: 'bash pnpm test [status: completed]' },
+        provenance: { role: 'inject', label: 'tool-tasks' },
+        form: 'notice',
+      } as never}
+      />,
+    )
+    expect(view.container.querySelector('[data-context-summary]')?.textContent)
+      .toBe('bash pnpm test [status: completed]')
+    expect(view.container.querySelector('[data-context-injection-body]')).toBeNull()
+  })
+
+  it('a notice without its account falls back to the opaque body', () => {
+    const view = render(
+      <MessageItem t={t} node={{
+        kind: 'context', seq: 3, content: [{ type: 'text', text: 'notice prose' }],
+        source: { kind: 'plugin', plugin: 'tool-tasks', form: 'notice' },
+        provenance: { role: 'inject', label: 'tool-tasks' },
+        form: 'notice',
+      } as never}
+      />,
+    )
+    expect(view.container.querySelector('[data-context-summary]')).toBeNull()
+    fireEvent.click(view.getByRole('button', { name: /^上下文注入\s*tool-tasks$/ }))
+    expect(view.container.querySelector('[data-context-fields]')).not.toBeNull()
+  })
+
+  it('a relay names the agent that sent it above what it said', () => {
+    const view = render(
+      <MessageItem t={t} node={{
+        kind: 'context',
+        seq: 3,
+        content: [{ type: 'text', text: 'child report body' }],
+        source: { kind: 'subagent-report', form: 'relay', senderSessionId: 'child-7' },
+        provenance: { role: 'inject', label: 'subagent-report' },
+        form: 'relay',
+      } as never}
+      />,
+    )
+    fireEvent.click(view.getByRole('button', { name: /^上下文注入\s*subagent-report$/ }))
+    expect(view.container.querySelector('[data-context-relay-sender]')?.textContent).toBe('来自会话 child-7')
+    expect(view.container.querySelector('[data-context-text]')?.textContent).toBe('child report body')
+  })
+
+  it('a recall reports how much of each source session survived the read', () => {
+    // Recalled context is bounded on the way in, so hiding the omitted count
+    // would overstate what the model received.
+    const view = render(
+      <MessageItem t={t} node={{
+        kind: 'context',
+        seq: 3,
+        content: [{ type: 'text', text: 'recalled material' }],
+        source: {
+          kind: 'session-reference',
+          form: 'recall',
+          version: 1,
+          references: [
+            { label: '重构 loader', retainedMessages: 18, omittedMessages: 42, truncated: true },
+            { label: '修 CI', retainedMessages: 3, omittedMessages: 0, truncated: false },
+          ],
+        },
+        provenance: { role: 'recall', label: '重构 loader, 修 CI' },
+        form: 'recall',
+      } as never}
+      />,
+    )
+    fireEvent.click(view.getByRole('button', { name: /^跨会话召回\s*重构 loader, 修 CI$/ }))
+    const rows = [...view.container.querySelectorAll('[data-context-recalls] li')].map(node => node.textContent)
+    expect(rows).toEqual(['重构 loader保留 18 条 · 省略 42 条已截断', '修 CI保留 3 条 · 省略 0 条'])
+    expect(view.container.querySelector('[data-context-text]')?.textContent).toBe('recalled material')
+  })
+
   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} />,