refactor: 拆分 3 个过大 ACP 文件为模块化子文件（每个 <500 行）

通过 4 阶段 workflow（分析 → 计划 → 重构 → 验证）将 3 个超大的 ACP
源文件拆分为 28 个模块化子文件，每个均严格小于 500 行，且完整保留
所有公共 API（barrel 模式重导出）。

变更概要：
- packages/acp-link/src/server.ts: 1800 → 20 行（barrel），新增 11 个子模块
  （server/types、payload-decode、permission-mode、runtime-state、dispatch、
  handlers-agent、handlers-session、acp-client、client-send、start-server、
  testing-internals）
- src/services/acp/agent.ts: 1297 → 33 行（barrel），新增 9 个子模块
  （agent/AcpAgent、sessionTypes、permissionMode、configOptions、promptQueue、
  internalAccessors、createSessionMethod、sessionLifecycle、promptFlow）
- src/services/acp/bridge.ts: 1516 → 29 行（barrel），新增 8 个子模块
  （bridge/types、paths、contentBlocks、toolInfo、toolResults、modelUsage、
  notifications、forwarding）

验证：
- bun run precheck 全通过（typecheck + lint + 5851 tests）
- ACP service tests: 176 pass / 0 fail
- ACP link tests: 47 pass / 0 fail
- 所有外部消费者（entry.ts、permissions.ts、__tests__/）的 import 路径不变
- 测试文件零修改

迁移计划详见 docs/acp-refactor-plan.md。

Co-Authored-By: glm-5.2 <zai-org@claude-code-best.win>

src/services/acp/agent/internalAccessors.ts

@@ -0,0 +1,54 @@
+/**
+ * Internal accessors for AcpAgent private fields and session-state helpers,
+ * shared across the prototype-augmentation modules (createSessionMethod /
+ * sessionLifecycle / promptFlow).
+ *
+ * AcpAgent's `conn` and `clientCapabilities` fields are declared `private`
+ * on the shell class. TS-only privacy (no #) means bracket access still
+ * works at runtime, but we cast through `unknown` to keep tsc strict happy
+ * without widening the public API surface of the class.
+ */
+import type {
+  AgentSideConnection,
+  ClientCapabilities,
+} from '@agentclientprotocol/sdk'
+import type { AcpAgent } from './AcpAgent.js'
+import type { AcpSession } from './sessionTypes.js'
+
+type AcpAgentInternals = {
+  conn: AgentSideConnection
+  clientCapabilities: ClientCapabilities | undefined
+}
+
+export function getConnection(agent: AcpAgent): AgentSideConnection {
+  return (agent as unknown as AcpAgentInternals).conn
+}
+
+export function readClientCapabilities(
+  agent: AcpAgent,
+): ClientCapabilities | undefined {
+  return (agent as unknown as AcpAgentInternals).clientCapabilities
+}
+
+/**
+ * Update the session's current mode/model id based on the configId.
+ *
+ * This logic was originally the private `AcpAgent.syncSessionConfigState`
+ * method on the shell class. It is called by the prototype-augmented
+ * `updateConfigOption` (sessionLifecycle.ts) and `setSessionConfigOption`
+ * (promptFlow.ts). Moving it here keeps it next to its only callers and
+ * avoids the `noUnusedPrivateClassMembers` false positive that the
+ * cast-based access would otherwise trigger on the shell.
+ */
+export function syncSessionConfigState(
+  _agent: AcpAgent,
+  session: AcpSession,
+  configId: string,
+  value: string,
+): void {
+  if (configId === 'mode') {
+    session.modes = { ...session.modes, currentModeId: value }
+  } else if (configId === 'model') {
+    session.models = { ...session.models, currentModelId: value }
+  }
+}