Clarify where stuff happens in the CI workflow

.forgejo/workflows/continuous-integration.yaml

@@ -14,8 +14,18 @@ jobs:
       image: cimg/rust:1.85-node
     defaults:
       run:
+        # set the default working directory for each `run` step, relative to the
+        # workspace directory. this default only affects `run` steps (and if we
+        # tried to set the `working-directory` label for any other kind of step,
+        # it wouldn't be recognized anyway)
         working-directory: app-proto
     steps:
+      # check out the repository to the workspace directory. in the action
+      # description, you can get and set the workspace directory through the
+      # `github.workspace` and `env.GITHUB_WORKSPACE` labels, which always have
+      # the same value. in the container, you can get the workspace directory
+      # from the `GITHUB_WORKSPACE` environment variable
       - uses: https://code.forgejo.org/actions/checkout@v4
+      
       - uses: ./.forgejo/setup-trunk
       - run: RUSTFLAGS='-D warnings' cargo test