Resolve git outside the session PATH

A desktop launch inherits a `PATH` that holds none of the user's own
install locations, so `Command::new("git")` finds nothing usable, or on
macOS finds Apple's shim and only prompts to install the developer
tools.

Resolve git to an absolute path instead, searching `PATH` first and the
usual install locations after, and cache the result for the process.
`rad` is looked for the same way, which also fixes the CLI check for a
Homebrew or Nix install.

Cargo.lock

@@ -4080,6 +4080,7 @@ dependencies = [
  "thiserror 2.0.19",
  "ts-rs",
  "url",
+ "which",
 ]
 
 [[package]]
@@ -6605,6 +6606,15 @@ version = "0.1.12"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a28ac98ddc8b9274cb41bb4d9d4d5c425b6020c50c46f25559911905610b4a88"
 
+[[package]]
+name = "which"
+version = "8.0.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8f3ef584124b911bcc3875c2f1472e80f24361ceb789bd1c62b3e9a3df9ff43c"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "winapi"
 version = "0.3.9"