not applying yaml merges

[clux]

we need to do .apply_merge() on serde_yaml::Value to get support for yaml merge keys (i.e. <<:).

this is supported in serde_yaml as seen by their tests: https://github.com/dtolnay/serde-yaml/blob/bb7e683593c2e040a8fff9a2191cae3b65ef870f/tests/test_value.rs#L62-L99

this does mean we need to temporarily store the value in yaml format, then serialize so serde_json can deserialize, but should not be a big change