Default config file path: $XDG_CONFIG_HOME/runex/config.toml,
falling back to ~/.config/runex/config.toml when XDG_CONFIG_HOME is unset.
All platforms use this same resolution order.
Overridden by RUNEX_CONFIG env var.
Read a config file into a string with the same safety guarantees as load_config:
single fd for metadata+read (no TOCTOU), rejects symlinks at final path component on
Unix, rejects non-regular files (FIFO / device nodes), and enforces the 10 MB size cap.