Make prune and remote nuke fail when a delete fails (closes #290)

prune, and the prune step of snapshot create --prune, counted a blob
it could not delete and still exited 0. It now prints its summary or
--json document and then returns an error, which replaces the
"Failed to delete N blob(s)" line.

remote nuke only warned when a snapshot's metadata could not be
deleted, advised vaultik prune, which never removes metadata, and then
reported the store empty. It now stops with an error, does not report
the store empty, and says to run vaultik remote nuke --force again. It
checks that the local index belongs to the configured destination
before deleting anything, and that error carries no such advice. A
successful nuke no longer advises prune.

Model: opus-5-5

TODO.md

@@ -22,6 +22,17 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-13: Made `prune` and `remote nuke` exit non-zero when a delete
+  fails ([issue #290](https://git.eeqj.de/sneak/vaultik/issues/290)).
+  `prune`, and the prune step of `snapshot create --prune`, counted a
+  blob it could not delete and still exited 0; it now prints its summary
+  or `--json` document and then fails. `remote nuke` warned about a
+  snapshot's metadata it could not delete, said to run `vaultik prune`,
+  which never removes metadata, and then reported the store empty. It
+  now fails without that report and says to run `vaultik remote nuke
+  --force` again, and a successful nuke no longer advises `prune` for
+  blobs it deletes itself.
+
 - 2026-10-13: Made `snapshot remove` fail for a snapshot that does not
   exist ([issue #293](https://git.eeqj.de/sneak/vaultik/issues/293)).
   The command never checked the ID: it reported the snapshot removed