Make path-based config loading public

[gkennos]

Pre-flight

• I searched existing issues and this has not been requested before.

Problem or motivation

_load_from_path(path) is private, but loading a stack config from an explicit path is a genuine application need

e.g. groundworkers has bootstrap.py::load_stack_config_from_path, a near-line-for-line reimplementation

The copy does one thing better: it formats ValidationError into a ConfigurationError naming the offending field paths, where upstream lets the raw pydantic error propagate.

Proposed solution

Promote it, taking the improved error formatting from groundworkers while we are at it

def load_stack_config_from_path(path: str | Path) -> StackConfig: ...

Keep load_stack_config() as the no-argument, CONFIG_PATH-driven entry point for application code

Alternatives considered

No response