Function default_history_dir 

pub fn default_history_dir() -> PathBuf

The centralized directory oneharness run history is written to, shared across every skilltest invocation so past runs accumulate in one reviewable place (rather than scattering per project the way oneharness’s own default would).

Resolution: SKILLTEST_HISTORY_DIR if set (the escape hatch tests use); otherwise <state dir>/skilltest/oneharness-history, where the state dir is $XDG_STATE_HOME or, failing that, $HOME/.local/state — the same convention oneharness follows across the Linux/macOS matrix. A last-resort fallback uses the temp dir so the path is always absolute.