Function find_agents_root 

pub fn find_agents_root(
    explicit: Option<&Path>,
) -> Result<MarsContext, MarsError>

Find mars project root by walking up from start path to filesystem root.

For context commands (add, sync, etc.), this walks from the start path (cwd or --root) until it finds a mars.toml. Walk-up continues to filesystem root — git boundaries do not stop the walk.

If --root is provided, it sets the walk-up start path, not a direct target.