are stack configs not uploaded along with the stack to app.pulumi.com? i.e. should we be adding the stack specific yaml to git

There is no persistent configuration associated with a stack in the service - it is provided as input on each update based on the values that are available locally. For long lived stacks that are being collaborated on by multiple users, you will likely want to commit the

Pulumi.<stack>.yaml

to Git. For short-lived or individual development stacks, you will not need to.