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. A test checks the --json help of both commands.

Model: opus-5-5

README.md

@@ -359,7 +359,9 @@ on the destination in one go, use `vaultik remote nuke --force`.
 * `--local-only`: Skip remote cleanup; only touch the local index
 * `--dry-run`: Show what would be deleted without deleting
 * `--force`: Skip confirmation prompt
-* `--json`: Output result as JSON
+* `--json`: Output result as JSON. Also skips the confirmation prompt, as
+  `--force` does, so `snapshot remove --json <snapshot-id>` removes without
+  asking.
 
 **`snapshot restore`**: Restore files from a backup snapshot.
 * Requires `VAULTIK_AGE_SECRET_KEY` environment variable
@@ -383,7 +385,8 @@ manifests — network cost scales with the number of snapshots. `snapshot
 create --prune` runs the same cleanup automatically; this is the
 manual entry point for the same work.
 * `--force`: Skip confirmation prompt
-* `--json`: Output stats as JSON
+* `--json`: Output stats as JSON. Also skips the confirmation prompt, as
+  `--force` does, so `prune --json` deletes unreferenced blobs without asking.
 
 **`info`**: Display system configuration, storage settings, encryption
 recipients, and local database statistics.