Never write fetch's temp file into an existing file (closes #115)

downloadFile opened the temp file with os.Create, which opens and
empties a file already at that name. If that file was a hard link to a
file outside the destination directory, fetch overwrote the outside
file. It now removes whatever is at the temp name, which removes only
that name, and creates the temp file with O_EXCL, so the create fails if
anything is still or again there. A leftover temp file from an
interrupted run is still replaced. The new test puts a hard link at the
temp name and checks that fetch succeeds and the outside file is
unchanged. The command name is now the constant cmdFetch, like the other
command names, because lint requires it once a third test uses it.

Model: opus-5-5

TODO.md

@@ -24,6 +24,9 @@ only thing left of the `chore/align-repo-policies` branch is the list below.
 
 # Completed Steps
 
+- 2026-10-03: `fetch` removes whatever sits at a file's temp name and then
+  creates the temp file only if that name is free, so a hard link left there
+  cannot make it write into a file outside the destination directory (#115)
 - 2026-10-03: `fetch` refuses any manifest path that runs through a symlink
   already in the destination directory, checked before each of its writes
   (directories, temp file, rename), so such a symlink cannot send a write