refactor(compaction): replay the derived system head as a message

The session already derives the protected system head as a Message, and both adapters accept leading system history. Passing it through a separate SummarizationInput.system string unnecessarily flattens that value and rebuilds the same wire message in the adapter. Prepend the derived head to messages and remove textContent, the separate field, and GenerateOptions.system plumbing from the summarizer.

Keep range selection, shadowed seq accounting, session head protection, routed tools, image references, target policy, and the model-visible compaction instruction unchanged. Empty-content heads still derive to null and contribute no request message, but their surface node remains protected. Update subclass consumers/tests, EN/ZH package and subsystem prose, and the existing system-prompt surface owning note with refreshed pairing records.

Evidence: pnpm exec vitest run packages/compaction/compaction-basic/tests packages/llm/llm-deepseek/tests/serialize.spec.ts packages/llm/llm-pi-ai/tests/context.spec.ts --coverage --coverage.include='packages/compaction/compaction-basic/src/region.ts' --coverage.include='packages/compaction/compaction-basic/src/summarizer.ts' passed 203 tests in 6 files; both changed sources have 100% statements, branches, functions, and lines. Region-to-default-summarizer cases pin exact prefix and tools for nonempty Unicode/multiline, empty, and absent heads. DeepSeek JSON byte equality and pi-ai context equality pin leading-message vs separate-system equivalence on text and image-capable conversion paths.

pnpm run doc-sync passed all 33 gates including doc-typecheck, documentation build, translation pairing and model-experience checks. git diff --check passed. Own dependencies installed with pnpm install --frozen-lockfile. An initial test iteration used a nonexistent ctx.dispose teardown on the in-memory fixture; corrected to its existing fixture lifecycle and reran successfully. No runtime/model behavior, normalizer marker, main worktree, push, or rebase changes.

packages/compaction/compaction-basic/tests/compaction-basic.spec.ts

@@ -660,8 +660,8 @@ describe('pressure measurement and retention', () => {
       role: 'system',
       content: [{ type: 'text', text: 'SYSTEM HEAD' }],
     })
-    expect(compact.calls[0]!.input.system).toBe('SYSTEM HEAD')
-    expect(summarizedText(compact.calls[0]!.input)).not.toContain('SYSTEM HEAD')
+    expect(compact.calls[0]!.input.messages[0]).toEqual(session.deriveMessages()[0])
+    expect(compact.calls[0]!.input.messages.filter(message => message.role === 'system')).toHaveLength(1)
   })
 
   it('declines when only the system head precedes the retained tail', () => {
@@ -947,13 +947,11 @@ describe('compaction region transaction', () => {
       reason: 'resume',
     })
     const nodes = session.surface.nodes
+    const prefix = session.deriveMessages().slice(0, 3)
     await compact.compactRegion(nodes[1]!, nodes[2]!, agent(session, MODEL), SIGNAL)
 
     const { input } = compact.calls[0]!
-    expect(input.system).toBe('CONVERSATION SYSTEM')
-    expect(input.tools).toEqual(tools)
-    expect(summarizedText(input)).toContain('fixture user 1')
-    expect(summarizedText(input)).not.toContain('CONVERSATION SYSTEM')
+    expect(input).toEqual({ messages: prefix, tools })
   })
 
   it('omits the summarizer system prompt for an empty system head or a system-less surface', async () => {
@@ -962,12 +960,14 @@ describe('compaction region transaction', () => {
     const emptyNodes = emptyHead.surface.nodes
     expect(emptyHead.eventAt(emptyNodes[0]!)?.type).toBe('system/message')
     await compact.compactRegion(emptyNodes[1]!, emptyNodes[2]!, agent(emptyHead, MODEL), SIGNAL)
-    expect(compact.calls[0]!.input).not.toHaveProperty('system')
+    expect(compact.calls[0]!.input.messages[0]).toMatchObject({ role: 'user' })
+    expect(compact.calls[0]!.input.messages.some(message => message.role === 'system')).toBe(false)
+    expect(emptyHead.surface.nodes[0]).toBe(emptyNodes[0])
 
     const headless = conversation(3)
     const nodes = headless.surface.nodes
     await compact.compactRegion(nodes[0]!, nodes[1]!, agent(headless, MODEL), SIGNAL)
-    expect(compact.calls[1]!.input).not.toHaveProperty('system')
+    expect(compact.calls[1]!.input.messages.some(message => message.role === 'system')).toBe(false)
     expect(compact.calls[1]!.input.messages[0]).toMatchObject({ role: 'user' })
   })
 
@@ -1259,6 +1259,27 @@ async function summarizerHarness(
 }
 
 describe('default one-shot summarizer', () => {
+  it.each([undefined, '', 'SYSTEM HEAD\n精确前缀\n'])('preserves the routed prefix through region summarization with system %j', async (system) => {
+    const { adapter, compact } = await summarizerHarness([{ type: 'text', text: 'summary' }])
+    const session = conversation(3, undefined, system)
+    const tools = [{ name: 'do_thing', description: 'd', parameters: { type: 'object' } }]
+    session.append('request/header', {
+      header: { config: { provider: MODEL, model: MODEL }, tools },
+      reason: 'resume',
+    })
+    const nodes = [...session.surface.nodes]
+    const start = system === undefined ? 0 : 1
+    const prefix = session.deriveMessages().slice(0, system ? 3 : 2)
+    const result = await compact.compactRegion(nodes[start]!, nodes[start + 1]!, agent(session, MODEL), SIGNAL)
+
+    expect(adapter.lastOptions).not.toHaveProperty('system')
+    expect(adapter.lastOptions?.tools).toEqual(tools)
+    expect(adapter.lastOptions?.messages.slice(0, -1)).toEqual(prefix)
+    expect(adapter.lastOptions?.messages.at(-1)).toMatchObject({ role: 'user' })
+    expect(result.shadowedSeqs).toEqual(nodes.slice(start, start + 2))
+    if (system !== undefined) expect(session.surface.nodes[0]).toBe(nodes[0])
+  })
+
   it('requires complete raw output when a subclass marks one local LLM stream call', () => {
     expectTypeOf<{
       summary: ContentBlock[]
@@ -1327,16 +1348,16 @@ describe('default one-shot summarizer', () => {
       ],
       source: { kind: 'plugin', plugin: 'test' },
     })
+    const system = createSystemMessage('REPLAYED SYSTEM', SYSTEM_PROMPT_PLUGIN)
     await compact.runSummarize({
-      system: 'REPLAYED SYSTEM',
       tools,
-      messages: [prefix],
+      messages: [system, prefix],
     }, agent(conversation(1), MODEL))
 
-    expect(adapter.lastOptions?.system).toBe('REPLAYED SYSTEM')
+    expect(adapter.lastOptions).not.toHaveProperty('system')
     expect(adapter.lastOptions?.tools).toEqual(tools)
     const messages = adapter.lastOptions?.messages ?? []
-    expect(messages[0]).toEqual(prefix)
+    expect(messages.slice(0, -1)).toEqual([system, prefix])
     const last = messages.at(-1)?.content[0]
     const lastText = last?.type === 'text' ? last.text : ''
     expect(lastText).toContain('Write concise English engineering prose.')
@@ -1368,9 +1389,9 @@ describe('default one-shot summarizer', () => {
       source: { kind: 'plugin', plugin: 'test' },
     })
 
+    const system = createSystemMessage('WARM SYSTEM', SYSTEM_PROMPT_PLUGIN)
     const output = await compact.runSummarize({
-      system: 'WARM SYSTEM',
-      messages: [prefix],
+      messages: [system, prefix],
     }, agent(conversation(1), 'fallback'))
 
     expect(output).toMatchObject({
@@ -1382,9 +1403,9 @@ describe('default one-shot summarizer', () => {
       provider: 'policy-summary',
       model: 'policy-summary',
       maxTokens: 222,
-      system: 'WARM SYSTEM',
     })
-    expect(policyAdapter.lastOptions?.messages[0]).toEqual(prefix)
+    expect(policyAdapter.lastOptions).not.toHaveProperty('system')
+    expect(policyAdapter.lastOptions?.messages.slice(0, -1)).toEqual([system, prefix])
   })
 
   it('resolves the latest routed provider/model before the AgentOptions pair', async () => {