Skip files of an unreadable blob under restore --skip-errors (closes #218)

A blob that failed to download ended `snapshot restore` even with
--skip-errors, after restoring whichever files came first. The download
error now goes through the same per-file handling as any other restore
error, once for every pending file that references the blob. With
--skip-errors those files are reported as failed, the rest are
restored, and the command still exits non-zero. Without the flag the
restore still aborts; the error now also names one affected file and
suggests --skip-errors. A cancelled restore still ends at once. The
--skip-errors help text and README now limit the packing and storage
caveat to snapshot creation.

Model: opus-5-5

TODO.md

@@ -22,6 +22,14 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-06: Made `snapshot restore --skip-errors` skip the files that
+  need a blob it cannot download
+  ([issue #218](https://git.eeqj.de/sneak/vaultik/issues/218)). A missing
+  or damaged blob ended the restore even with the flag, after restoring
+  whichever files happened to come first. Every file that needs such a
+  blob is now reported as failed, the rest are restored, and the command
+  still exits non-zero. Without the flag the blob error still aborts.
+
 - 2026-10-06: Re-vendored the canonical files from `sneak/prompts` at
   `dd4027b` ([issue #213](https://git.eeqj.de/sneak/vaultik/issues/213)).
   Linting and testing are now the `lint` and `test` phases of the