POSIX permits short syscalls (incl. open/read/stat) to be interrupted
by a signal and surface as EINTR. Optional sync git metadata helpers
(used by the status line) propagated the raw error and crashed the
agent. Add a small bounded retry around the sync stat/read calls and
classify persistent EINTR as 'metadata unavailable' so optional reads
fall back to undefined instead of throwing.
Fixes#899
can1357