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 on
stderr carries the follow-up, with the snapshot ID as a field.

The follow-up, in the warning, the README and the command's help, said
`vaultik prune` would finish the cleanup, but `prune` never removes
snapshot metadata. They now say to run `vaultik snapshot remove` for the
snapshot again once the destination store is reachable.

Model: opus-5-5

internal/vaultik/snapshot.go

@@ -1110,6 +1110,12 @@ type RemoveResult struct {
 // just-removed snapshot left behind on the destination store.
 const pruneCommandHint = "vaultik prune"
 
+// snapshotRemoveCommandHint is the command suggested, with the
+// snapshot's ID, when a remove could not reach the destination store:
+// running it again removes the snapshot's metadata there, which
+// `vaultik prune` never does.
+const snapshotRemoveCommandHint = "vaultik snapshot remove"
+
 // RemoveSnapshot removes a snapshot from the local index database and,
 // unless LocalOnly is set, also strips the snapshot's metadata from the
 // destination store. Blobs are NOT touched: removing a snapshot's
@@ -1146,7 +1152,7 @@ func (v *Vaultik) RemoveSnapshot(
 	}
 
 	if !opts.LocalOnly {
-		result.RemoteRemoved = v.removeSnapshotRemote(snapshotID)
+		result.RemoteRemoved = v.removeSnapshotRemote(snapshotID, opts)
 	}
 
 	if v.SnapshotManager != nil {
@@ -1229,9 +1235,11 @@ func (v *Vaultik) confirmRemoveSnapshot(snapshotID string, opts *RemoveOptions)
 // removeSnapshotRemote strips the snapshot's metadata from the
 // destination store, warning and proceeding on failure: the local-DB
 // removal has already happened, so the user is told the remote half
-// didn't finish and can retry with `vaultik prune` once the destination
-// store is reachable. Returns true when the remote removal succeeded.
-func (v *Vaultik) removeSnapshotRemote(snapshotID string) bool {
+// didn't finish and to run `vaultik snapshot remove` for the snapshot
+// again once the destination store is reachable (`vaultik prune` never
+// removes snapshot metadata). Returns true when the remote removal
+// succeeded.
+func (v *Vaultik) removeSnapshotRemote(snapshotID string, opts *RemoveOptions) bool {
 	log.Info("Removing snapshot metadata from remote storage",
 		"snapshot_id", snapshotID)
 
@@ -1239,13 +1247,17 @@ func (v *Vaultik) removeSnapshotRemote(snapshotID string) bool {
 
 	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 '"+snapshotRemoveCommandHint+"' with the snapshot's ID "+
+			"again once the remote is reachable",
+			"snapshot_id", snapshotID, "error", err)
 
-		if v.UI != nil {
+		// The UI writes to stdout, which under --json holds only the
+		// document; the log record above is the warning on stderr.
+		if v.UI != nil && !opts.JSON {
 			v.UI.Warningf("Could not remove snapshot metadata from remote: "+
-				"%v. Run '%s' once the remote is reachable to finish cleanup.",
-				err, pruneCommandHint)
+				"%v. Run '%s %s' again once the remote is reachable.",
+				err, snapshotRemoveCommandHint, snapshotID)
 		}
 
 		return false