refactor: 删除外置 skills 系统，全部工具转为内置

删除:
- resources/skills/ (9个外置 skill, bash/python/node 脚本)
- src/tools/registry.rs (SkillRegistry 加载/执行)
- src/tools/parser.rs (SKILL.md 解析)
- src/skills_importer.rs (skills 导入)
- cli.rs Skills 子命令
- main.rs cmd_skills / global_skills_dir

保留:
- src/tools/builtin.rs (内置工具注册 + 审批流程)
- src/tools/weather.rs (天气查询)
- src/tools/skill.rs (精简为 SkillSpec/RiskLevel/SkillResult/ExecutionContext)
- src/tools/approval.rs (审批管理器)

main.rs 简化: 移除 SkillRegistry 加载, executor 只走 BuiltinRegistry

Cargo.lock

@@ -138,6 +138,18 @@ dependencies = [
  "stable_deref_trait",
 ]
 
+[[package]]
+name = "async-compression"
+version = "0.4.42"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e79b3f8a79cccc2898f31920fc69f304859b3bd567490f75ebf51ae1c792a9ac"
+dependencies = [
+ "compression-codecs",
+ "compression-core",
+ "pin-project-lite",
+ "tokio",
+]
+
 [[package]]
 name = "async-trait"
 version = "0.1.89"
@@ -393,6 +405,23 @@ version = "1.0.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1d07550c9036bf2ae0c684c4297d503f838287c83c53686d05370d0e139ae570"
 
+[[package]]
+name = "compression-codecs"
+version = "0.4.38"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ce2548391e9c1929c21bf6aa2680af86fe4c1b33e6cea9ac1cfeec0bd11218cf"
+dependencies = [
+ "compression-core",
+ "flate2",
+ "memchr",
+]
+
+[[package]]
+name = "compression-core"
+version = "0.4.32"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cc14f565cf027a105f7a44ccf9e5b424348421a1d8952a8fc9d499d313107789"
+
 [[package]]
 name = "concurrent-queue"
 version = "2.5.0"
@@ -3190,12 +3219,17 @@ version = "0.6.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "4cfcf7e2740e6fc6d4d688b4ef00650406bb94adf4731e43c096c3a19fe40840"
 dependencies = [
+ "async-compression",
  "bitflags",
  "bytes",
+ "futures-core",
  "futures-util",
  "http",
  "http-body",
+ "http-body-util",
  "pin-project-lite",
+ "tokio",
+ "tokio-util",
  "tower",
  "tower-layer",
  "tower-service",