fix: harden Cub git and tempdir handling

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

local/recipes/system/cub/source/cub-lib/Cargo.toml

@@ -26,7 +26,8 @@ pkgar-core = { version = "0.2.2", optional = true }
 pkgar-keys = { version = "0.2.2", optional = true }
 
 # HTTP for source fetching
-reqwest = { version = "0.12", default-features = false, features = ["blocking", "rustls-tls"], optional = true }
+reqwest = { version = "0.12", default-features = false, features = ["blocking", "rustls-tls", "json"], optional = true }
+serde_json = "1"
 
 [features]
 default = ["full"]