prefer-pathlib-over-os-path

# Paths `pathlib.Path` reads better than string joining and carries its own helpers. 1. Take `Path` objects in signatures that accept a path, and accept `str` only at the edge where a caller hands one in. 2. Build a child path with the `/` operator: `root / "data" / "catalog.json"`. 3. Read and write through `Path.read_text()` and `Path.write_text()` with an explicit `encoding="utf-8"` rather than opening a file by hand. 4. Use `Path.resolve()` once, at the boundary, and pass the resolved path down instead of resolving it again in every function that receives it. A signature that accepts both a string and a Path should say so in its annotation (`str | Path`) so a reader knows which forms are handled.