feat(agent): 实现 ContextSlot fork/merge

- 新增 MergeStrategy 枚举（#[non_exhaustive]，为 Phase 16 Summarize 预留）
- 新增 ContextSlot::fork() 从父 slot 派生独立子 slot（Full / Focused 策略）
- 新增 ContextSlot::merge() 将子 slot 消息合入父 slot（Append / Replace 策略）
- 防御性检查：禁止 self-merge、跨 session merge、合并到 Readonly slot
- 重构 AgentSession::derive_slot 复用 fork() 消除重复代码
- 新增 9 个内联测试覆盖 fork/merge happy path 与 error path
- agent.rs 追加 MergeStrategy re-export（agcore::agent::MergeStrategy 路径可用）

src/agent/session.rs

@@ -16,8 +16,11 @@ use futures_core::Stream;
 
 use crate::agent::agent::Agent;
 use crate::agent::context::{
-    ContextSlot, DeriveStrategy, SlotConfig, SlotMode, SlotSource,
+    ContextSlot, DeriveStrategy, SlotConfig, SlotMode,
 };
+// SlotSource 仅在 `mod tests` 中使用（通过 `use super::*;` 引入），lib 主体保留以避免测试 import 变更。
+#[allow(unused_imports)]
+use crate::agent::context::SlotSource;
 use crate::agent::error::AgentError;
 use crate::agent::runtime::RuntimeBundle;
 use crate::agent::session_memory::SessionMemory;
@@ -224,38 +227,9 @@ impl AgentSession {
             .slots
             .get(parent_id)
             .ok_or_else(|| AgentError::SlotNotFound(parent_id.to_string()))?;
-
-        let parent_messages = parent.messages.clone();
-        let (messages, focused_cfg) = match &strategy {
-            DeriveStrategy::Full => (parent_messages, None),
-            DeriveStrategy::Focused(cfg) => {
-                let filtered = ContextSlot::filter_focused(&parent_messages, cfg);
-                (filtered, Some(cfg.clone()))
-            }
-        };
-
-        let mode = match focused_cfg {
-            Some(cfg) => SlotMode::Focused(cfg),
-            None => SlotMode::Full,
-        };
-
-        let slot = ContextSlot::new(
-            &self.session_id,
-            &slot_id,
-            SlotConfig {
-                mode,
-                source: SlotSource::Derived {
-                    parent_id: parent_id.to_string(),
-                    strategy,
-                },
-                budget: Default::default(),
-                compact: true,
-            },
-        );
-        let mut slot = slot;
-        slot.messages = messages;
-        slot.save(&*self.resolve_store()).await?;
-        self.slots.insert(slot_id, slot);
+        let child = parent.fork(slot_id.clone(), strategy);
+        child.save(&*self.resolve_store()).await?;
+        self.slots.insert(slot_id, child);
         Ok(())
     }