refactor(core): 完成 v0.3.2 Cargo features 拆分基础设施

- 定义 16 个 features + 4 个快捷组合，default = ["full"] 保持向后兼容
- 12 个重型依赖 optional 化（tokio/reqwest/rusqlite 等）
- 全模块 #[cfg(feature)] 门控注入（llm/tools/memory/agent/engine）
- 将 LlmProvider trait 及关联类型移出 provider 模块归属 llm（ADR-1）
- 为 session.rs bundle() 方法添加 engine feature 门控
- 更新 12 个内部文件 + 4 个示例文件的 import 路径
- 向后兼容：provider.rs 保留 pub use 重导出老路径

examples/context_slot_demo.rs

@@ -15,7 +15,7 @@ use std::sync::Arc;
 use agcore::agent::{Agent, AgentBuilder, AgentSession};
 use agcore::llm::hooks::HookExecutor;
 use agcore::llm::mock::MockProvider;
-use agcore::llm::provider::LlmProvider;
+use agcore::llm::LlmProvider;
 use agcore::llm::types::message::{ContentBlock, Message};
 use agcore::llm::types::response_v2::{MessageResponse, StopReason};
 use agcore::llm::types::Usage;

examples/end_to_end.rs

@@ -18,7 +18,8 @@ use std::sync::Arc;
 use agcore::agent::{Agent, AgentBuilder, AgentSession};
 use agcore::llm::hooks::HookExecutor;
 use agcore::llm::mock::MockProvider;
-use agcore::llm::provider::{create_provider, LlmProvider, ProviderConfig, ProviderType};
+use agcore::llm::LlmProvider;
+use agcore::llm::provider::{create_provider, ProviderConfig, ProviderType};
 use agcore::llm::types::{Usage, message::{ContentBlock, Message}, response_v2::{MessageResponse, StopReason}};
 use agcore::memory::store::{MemoryStore, SqliteStore};
 use agcore::memory::types::{MemoryFilter, MemoryItem};

examples/quick_start.rs

@@ -6,7 +6,7 @@ use std::sync::Arc;
 use agcore::agent::{Agent, AgentBuilder, AgentSession};
 use agcore::llm::hooks::HookExecutor;
 use agcore::llm::mock::MockProvider;
-use agcore::llm::provider::LlmProvider;
+use agcore::llm::LlmProvider;
 use agcore::llm::types::{Usage, message::{ContentBlock, Message}, response_v2::{MessageResponse, StopReason}};
 use agcore::tools::{BaseTool, ToolContext, ToolError, ToolRegistry};
 use async_trait::async_trait;

examples/streaming_events_demo.rs

@@ -16,7 +16,7 @@ use std::sync::Arc;
 
 use agcore::llm::cycle::{CycleConfig, LlmCycle};
 use agcore::llm::mock::MockProvider;
-use agcore::llm::provider::LlmProvider;
+use agcore::llm::LlmProvider;
 use agcore::llm::types::Usage;
 use agcore::llm::types::message::{ContentBlock, Message};
 use agcore::llm::types::response_v2::{MessageResponse, StopReason, StreamEvent};