Write only the document to stdout from snapshot remove --json (closes #251)

When the destination store cannot be reached, `snapshot remove` still
removes the snapshot from the local index and warns. The warning went
through the UI, which writes to stdout, so under `--json` it landed
ahead of the document and `| jq` failed on a command that exited 0.
Under `--json` the UI warning is now skipped; the logger's warning
already goes to stderr.

The new test runs the command through `Entry` against a missing
destination directory and decodes stdout as exactly one JSON document.

Judgement call: under `--json` the stderr warning is the log record,
which names the error but not the `vaultik prune` follow-up.

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 remove --json` write only its document to
+  stdout when the destination store cannot be reached
+  ([issue #251](https://git.eeqj.de/sneak/vaultik/issues/251)). Its
+  warning that the snapshot's metadata was left on the destination store
+  went to stdout ahead of the document, breaking `| jq` on a command that
+  exited 0. Under `--json` the warning now reaches stderr only, through
+  the logger.
+
 - 2026-10-06: Made command output follow the README's stdout and stderr
   rules ([issue #224](https://git.eeqj.de/sneak/vaultik/issues/224)). The
   startup banner went to stdout, so a `completion` script or a