refactor: modularize CLI and optimize cache metadata lookups

- Extract error handling, path validation, and handlers into separate modules
- Add CacheCheckResult to pre-fetch metadata and avoid double lookups
- Deprecate legacy cache methods in favor of optimized alternatives
- Enable tokio fs feature for async file operations
- Remove debug profile from release build

Cargo.toml

@@ -13,12 +13,9 @@ reqwest = { version = "0.11", default-features = false, features = ["rustls-tls"
 serde = { version = "1.0.228", features = ["derive"] }
 serde_json = "1.0.145"
 thiserror = "2.0.11"
-tokio = { version = "1.48.0", features = ["rt-multi-thread", "macros", "sync", "time"] }
+tokio = { version = "1.48.0", features = ["rt-multi-thread", "macros", "sync", "time", "fs"] }
 toml = "0.8.19"
 walkdir = "2.5.0"
 
 [dev-dependencies]
 tempfile = "3.15"
-
-[profile.release]
-debug = true