fix: make atomic saves durable and preserve the file

writeAtomic renamed the temporary file over the target without syncing
either: on ext4 with delayed allocation a power cut just after a save
could leave a zero-length script, the exact failure autosave exists to
prevent. Sync the temp file before the rename and the directory after
it. While here, keep an existing file's permissions instead of forcing
0o644, and resolve symlinks first so saving through a link replaces
the file it points to, not the link itself (Dropbox/dotfile setups).

CHANGELOG.md

@@ -11,6 +11,10 @@ version is in the `VERSION` file.
 
 - The unsaved-changes marker is now updated atomically: an edit landing while a save
   finished could make the close confirmation unreliable.
+- Saving is now durable: the file is synced to disk before it replaces the old one (a
+  power cut just after a save could leave an empty script). An existing file keeps its
+  permissions, and saving through a symlink replaces the file it points to instead of
+  replacing the link.
 
 ## [1.4] - 2026-10-15