feat: skills import — 从其他工具复制 Skills 的交互式 TUI

新增:
- skills_importer.rs: 扫描已知路径 (Claude Code, Agent Skills,
  OpenCode, pi) 发现可导入的 Skill
- ias skills list: 列出可导入的 Skill
- ias skills import: 交互式多选 TUI (dialoguer MultiSelect)
  复制选中的 Skill 到 skills/ 目录

依赖: dialoguer + console + dirs

Cargo.toml

@@ -38,3 +38,6 @@ async-trait = "0.1"
 futures-util = "0.3"
 sqlx = { version = "0.8.6", features = ["runtime-tokio-native-tls", "postgres", "uuid", "chrono", "derive", "migrate"] }
 tracing-appender = "0.2.5"
+dialoguer = "0.12.0"
+console = "0.16.3"
+dirs = "6.0.0"