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,
which goes to stderr in every mode, now also says to run `vaultik
prune` once the destination store is reachable.

The new test runs the command through `Entry` against a missing
destination directory, decodes stdout as exactly one JSON document, and
finds the warning and the `vaultik prune` follow-up on stderr.

Model: opus-5-5

TODO.md

@@ -28,7 +28,8 @@ the tag exists and is exercised; what is left is merging `next` to
   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.
+  the logger, whose record now also says to run `vaultik prune` once the
+  destination store is reachable.
 
 - 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

internal/cli/entry_stdout_stderr_test.go

@@ -91,7 +91,8 @@ func TestEntryJSONFailureIsReportedOnStderr(t *testing.T) {
 // --json` against a destination directory that does not exist. The
 // command still exits 0 after removing the snapshot from the local
 // index. Its stdout must hold the document alone, with the warning
-// about the destination store on stderr.
+// about the destination store, and the `vaultik prune` follow-up, on
+// stderr.
 //
 //nolint:paralleltest // replaces os.Args, os.Stdout, os.Stderr and the xdg globals
 func TestEntrySnapshotRemoveJSONWarningIsOnStderr(t *testing.T) {
@@ -104,6 +105,7 @@ func TestEntrySnapshotRemoveJSONWarningIsOnStderr(t *testing.T) {
 	requireExactlyOneJSONDocument(t, stdout)
 	assert.Contains(t, stderr,
 		"Could not remove snapshot metadata from remote storage")
+	assert.Contains(t, stderr, "run 'vaultik prune'")
 }
 
 // writeMissingDestinationConfig builds a config whose destination

internal/vaultik/snapshot.go

@@ -1260,8 +1260,9 @@ func (v *Vaultik) removeSnapshotRemote(snapshotID string, opts *RemoveOptions) b
 
 	err := v.deleteRemoteSnapshotByKey(remoteKey)
 	if err != nil {
-		log.Warn("Could not remove snapshot metadata from remote storage",
-			"error", err)
+		log.Warn("Could not remove snapshot metadata from remote storage; "+
+			"run '"+pruneCommandHint+"' once the remote is reachable "+
+			"to finish cleanup", "error", err)
 
 		// The UI writes to stdout, which under --json holds only the
 		// document; the log record above is the warning on stderr.