Files under ~/.openharness/ — credentials, settings, session snapshots,
cron registry, memory index — were written with `Path.write_text()` in
truncating mode. A crash, SIGKILL, power loss, or out-of-disk error
during the write leaves a truncated file on disk; concurrent writers
clobber each other's updates; and the credentials file spent a brief
window at the default umask mode (commonly 0o644) before chmod-to-0600
ran.
Introduce `openharness.utils.fs.atomic_write_text` / `atomic_write_bytes`
which write to a same-directory temp file, fsync, apply the target mode
while the file is still private, and `os.replace` into place. Thread
them through all persistence writers. For read-modify-write on shared
files (credentials, settings, cron, memory index), pair atomic writes
with the existing `exclusive_file_lock` primitive so two `oh` processes
no longer race.
The generic lock helper moves from `openharness.swarm.lockfile` to
`openharness.utils.file_lock`. `swarm.lockfile` is retained as a thin
re-export so existing callers keep working.
Co-authored-by: José Maia <glitch-ux@users.noreply.github.com>
José Maia