refactor(llm): 删除手写 provider 实现与冗余依赖

- 删除 6 个手写 provider 文件、LlmProvider trait、动态分发、HTTP 客户端工厂与门面死代码
- llm 模块收敛为 rig 门面 + 提示词/解析/思考状态四个模块
- 移除 reqwest(0.12) 与 async-trait 直接依赖（futures-util 由流式消费保留）
- 依赖树确认无 rig-agent/fastembed/lancedb/milvus 等组件

Cargo.toml

@@ -32,8 +32,6 @@ dirs = "5.0"
 git2 = "0.20.3"
 which = "6.0"
 
-# HTTP client for LLM APIs
-reqwest = { version = "0.12", features = ["json", "rustls-tls", "stream"], default-features = false }
 tokio = { version = "1.35", features = ["full", "macros", "rt-multi-thread"] }
 
 # Error handling
@@ -57,7 +55,6 @@ tempfile = "3.9"
 sha2 = "0.10"
 hex = "0.4"
 textwrap = "0.16"
-async-trait = "0.1"
 futures-util = "0.3"
 serde_json = "1.0"
 atty = "0.2"