新增: MemoryAdapter trait + ContextManager 默认实现

crates/df-ai/src/memory_adapter.rs:
- MemoryAdapter trait: 15 个核心方法（写入/读取/压缩/生命周期）
- ContextManager 作为默认实现（行为零变化）
- 预留 VectorMemoryAdapter 接入点

lib.rs: pub mod memory_adapter 导出

crates/df-ai/src/memory_adapter.rs

@@ -0,0 +1,86 @@
+//! MemoryAdapter trait — 上下文记忆层抽象
+//!
+//! 将 ContextManager 的核心接口提取为 trait，支持多种记忆后端：
+//! - InMemoryAdapter（当前 ContextManager 的包装，默认实现）
+//! - VectorMemoryAdapter（未来：接入向量检索，支持语义记忆）
+//!
+//! 使用方式：`Box<dyn MemoryAdapter>` 替代 `ContextManager`。
+
+use crate::context::ContextManager;
+use crate::context_helpers::{ContextConfig, TrackedMessage};
+use crate::provider::ChatMessage;
+
+/// 上下文记忆层抽象接口
+///
+/// 覆盖 agentic loop / IPC 最常用的 15 个方法。
+/// 保留 `ContextManager` 作为默认实现（`InMemoryAdapter`），行为零变化。
+pub trait MemoryAdapter: Send + Sync {
+    // ── 写入 ──
+    fn push(&mut self, msg: ChatMessage);
+    fn clear(&mut self);
+    fn insert_at(&mut self, index: usize, message: ChatMessage);
+    fn replace_tool_result_content(&mut self, tool_call_id: &str, new_content: &str) -> bool;
+    fn pop_last_assistant_round(&mut self) -> bool;
+    fn truncate_after_user_message(&mut self, target_content: &str) -> Result<usize, ()>;
+    fn replace_last_active_user_content(&mut self, new_content: &str) -> Result<(), ()>;
+
+    // ── 读取 ──
+    fn build_for_request(&self, sys_tokens: u32) -> (Vec<ChatMessage>, bool);
+    fn all_messages_clone(&self) -> Vec<ChatMessage>;
+    fn len(&self) -> usize;
+    fn is_empty(&self) -> bool;
+    fn history_tokens(&self) -> u32;
+    fn budget_limit(&self) -> u32;
+    fn config(&self) -> &ContextConfig;
+
+    // ── 压缩 ──
+    fn compress_old_messages(&mut self, end: usize) -> Vec<ChatMessage>;
+    fn has_compressible_messages(&self, protect_start: usize) -> bool;
+    fn is_compressing(&self) -> bool;
+    fn set_compressing(&mut self, v: bool);
+
+    // ── 生命周期 ──
+    fn restore_from_messages(&mut self, messages: Vec<ChatMessage>);
+    fn take_topic_marker(&mut self) -> Option<String>;
+    fn messages_mut(&mut self) -> &mut [TrackedMessage];
+    fn iter(&self) -> Box<dyn Iterator<Item = &ChatMessage> + '_>;
+}
+
+/// ContextManager 作为默认 MemoryAdapter 实现
+impl MemoryAdapter for ContextManager {
+    fn push(&mut self, msg: ChatMessage) { self.push(msg); }
+    fn clear(&mut self) { self.clear(); }
+    fn insert_at(&mut self, index: usize, message: ChatMessage) { self.insert_at(index, message); }
+    fn replace_tool_result_content(&mut self, tool_call_id: &str, new_content: &str) -> bool {
+        self.replace_tool_result_content(tool_call_id, new_content)
+    }
+    fn pop_last_assistant_round(&mut self) -> bool { self.pop_last_assistant_round() }
+    fn truncate_after_user_message(&mut self, target_content: &str) -> Result<usize, ()> {
+        self.truncate_after_user_message(target_content)
+    }
+    fn replace_last_active_user_content(&mut self, new_content: &str) -> Result<(), ()> {
+        self.replace_last_active_user_content(new_content)
+    }
+
+    fn build_for_request(&self, sys_tokens: u32) -> (Vec<ChatMessage>, bool) {
+        self.build_for_request(sys_tokens)
+    }
+    fn all_messages_clone(&self) -> Vec<ChatMessage> { self.all_messages_clone() }
+    fn len(&self) -> usize { self.len() }
+    fn is_empty(&self) -> bool { self.is_empty() }
+    fn history_tokens(&self) -> u32 { self.history_tokens() }
+    fn budget_limit(&self) -> u32 { self.budget_limit() }
+    fn config(&self) -> &ContextConfig { self.config() }
+
+    fn compress_old_messages(&mut self, end: usize) -> Vec<ChatMessage> { self.compress_old_messages(end) }
+    fn has_compressible_messages(&self, protect_start: usize) -> bool { self.has_compressible_messages(protect_start) }
+    fn is_compressing(&self) -> bool { self.is_compressing() }
+    fn set_compressing(&mut self, v: bool) { self.set_compressing(v); }
+
+    fn restore_from_messages(&mut self, messages: Vec<ChatMessage>) { self.restore_from_messages(messages); }
+    fn take_topic_marker(&mut self) -> Option<String> { self.take_topic_marker() }
+    fn messages_mut(&mut self) -> &mut [TrackedMessage] { self.messages_mut() }
+    fn iter(&self) -> Box<dyn Iterator<Item = &ChatMessage> + '_> {
+        Box::new(self.iter())
+    }
+}