Toggle the highlighted agent as the persisted default.
If the highlighted agent is already the default, clears it.
Otherwise sets it as the new default. Disk I/O is offloaded to a
thread so the modal stays responsive. The help line shows a
transient confirmation/error message; the option list is rebuilt
in place so the (default) marker tracks the new state.
Robustness notes:
to_thread call is wrapped in try/except Exception so
an unexpected error inside the persistence functions
(e.g., tomli_w.dump raising a TypeError) is treated as
a normal save failure rather than killing the modal.query_one is guarded against NoMatches so a
user dismissing the modal mid-flight does not surface a
Textual callback error._refresh_options,
which builds the new options first and only swaps on
success. A failure mid-rebuild leaves the existing list
intact and shows an error in the help line.