[gc-stack-deploy] Run unit tests in GitHub Actions on branch push

[conservationtimothy]

Goal

Run gc-stack-deploy unit tests in GitHub Actions on every branch push.

Screenshots

Not applicable.

What I changed and why

I added a GitHub Actions workflow that runs make -C caprover/tests test-unit on push to any branch.

I also updated caprover/tests/README.md so the documentation matches the new CI path

How I convinced myself this is right

Pretty straightforward but I also locally ran

act push --job test-unit \
  --container-architecture linux/amd64 \
  -P ubuntu-latest=catthehacker/ubuntu:act-latest

What I'm not doing here

This does not add CapRover e2e, test-e2e, or make all to CI. It does not change the unit test suite itself.

LLM use disclosure

None