Document that --json skips the confirmation prompt (closes #268)

snapshot remove and prune delete without asking under --json, because a
prompt on stdout would break the JSON document. Their --json help and
README entries named only --force as skipping the prompt. Both now say
--json skips it too. One test checks the --json help of both commands;
another runs prune with --json, no --force and empty stdin, and checks
that the unreferenced blob is deleted and stdout holds only the JSON
document.

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
 
+- 2026-10-07: Documented that `--json` skips the confirmation prompt of
+  `snapshot remove` and `prune`
+  ([issue #268](https://git.eeqj.de/sneak/vaultik/issues/268)). Both
+  commands delete without asking under `--json`, since a prompt on stdout
+  would break the JSON document, but the help and the README described
+  only `--force` as skipping it. The `--json` help of both commands and
+  their README entries now say so.
+
 - 2026-10-07: Made a symlink whose target cannot be read stop the backup
   ([issue #269](https://git.eeqj.de/sneak/vaultik/issues/269)). It was
   left out of the snapshot with only a debug log line, even without