Introduce `nestedUnderJQPath`, enabling subtree extraction using jq-style path parsing. Add associated utility functions (`splitJQPath`, `peelRedundantPathLeaf`, `buildNestedTreeAtPath`) and test coverage. Extend `.env Codec` to support path-specific subtree merges. Update `WithFile` to accept an optional jqPath for targeted merges.
Codify the article's nested Config scenario to verify YAML base values are overridden by dotenv and finally by process environment variables.
Co-authored-by: Cursor <cursoragent@cursor.com>