Run ACP login from same cwd as agent server (#42038)

This makes it possible to do login via things like `cmd: "node", args:
["my-node-file.js", "login"]`

Also, that command will now use Zed's managed `node` instance.

Release Notes:

- ACP extensions can now run terminal login commands using relative
paths

crates/extension/src/extension_manifest.rs

@@ -173,6 +173,10 @@ pub struct AgentServerManifestEntry {
     /// cmd = "node"
     /// args = ["index.js", "--port", "3000"]
     /// ```
+    ///
+    /// Note: All commands are executed with the archive extraction directory as the
+    /// working directory, so relative paths in args (like "index.js") will resolve
+    /// relative to the extracted archive contents.
     pub targets: HashMap<String, TargetConfig>,
 }