convert and transform YAML documents in shell scripts using yq v4's eval expression syntax

Steps

1. Run a read/transform expression against a file with `yq eval '.a.b.c' file.yaml` (or the shorthand `yq e`), using jq-like path/pipe syntax rather than yq v3's flag-based CLI.
2. Update a value in place with an assignment expression, e.g. `yq eval '.a.b.c = "newvalue"' -i file.yaml`, where -i edits the file in place.
3. Filter or select elements with select(...), e.g. `yq eval '.a.b.c | select(. == "cat")' file.yaml`, mirroring jq's filtering idiom.
4. Evaluate an expression built dynamically at runtime (e.g., from an environment variable) using the eval(...) operator inside a larger expression, rather than string-interpolating shell variables directly into the yq expression.
5. Process multiple files together with `yq eval-all` (aliased `yq ea`) when a transformation needs to see more than one document at once, since plain eval processes documents independently.