Silence prune and snapshot removal status under --cron and --quiet (closes #294)

PruneBlobs, CleanupLocalSnapshots, RemoveSnapshot, RemoveAllSnapshots
and snapshot purge wrote their status lines straight to stdout, so
`snapshot create --cron --prune` printed on every successful run and
--quiet did not silence `prune`, `snapshot purge`, `snapshot remove` or
`remote nuke`. Those lines now go through internal/ui, which drops them
under --cron and --quiet; warnings, errors and --json documents still
print.

The confirmation prompts still write straight to stdout, as the
`database delete` prompt does, together with what they ask to delete:
the snapshot list before the purge prompt, and the blob count and size
before the prune prompt. With --force those lines are status lines.

The two purge test fixtures gained the UI writer production always sets.

Model: opus-5-5

internal/vaultik/prune.go

@@ -204,7 +204,7 @@ func (v *Vaultik) PruneBlobs(opts *PruneOptions) error {
 			return v.outputPruneBlobsJSON(result)
 		}
 
-		v.printlnStdout("No unreferenced blobs to remove.")
+		v.UI.Infof("No unreferenced blobs to remove.")
 
 		return nil
 	}
@@ -212,12 +212,17 @@ func (v *Vaultik) PruneBlobs(opts *PruneOptions) error {
 	log.Info("Found unreferenced blobs",
 		"count", len(unreferencedBlobs), "total_size", ubytes(totalSize))
 
-	if !opts.JSON {
-		v.stdoutf("Found %d unreferenced blob(s) totaling %s\n",
+	if opts.Force && !opts.JSON {
+		v.UI.Infof("Found %d unreferenced blob(s) totaling %s.",
 			len(unreferencedBlobs), ubytes(totalSize))
 	}
 
 	if !opts.Force && !opts.JSON {
+		// The count, the prompt and its answer bypass the UI: they are an
+		// interactive exchange the operator must see, so --quiet does not
+		// silence them.
+		v.stdoutf("Found %d unreferenced blob(s) totaling %s.\n",
+			len(unreferencedBlobs), ubytes(totalSize))
 		v.stdoutf("\nDelete %d unreferenced blob(s)? [y/N] ", len(unreferencedBlobs))
 
 		var confirm string
@@ -244,7 +249,7 @@ func (v *Vaultik) PruneBlobs(opts *PruneOptions) error {
 			return err
 		}
 	} else {
-		v.stdoutf("\nDeleted %d blob(s) totaling %s\n",
+		v.UI.Infof("Deleted %d blob(s) totaling %s.",
 			result.BlobsDeleted, ubytes(result.BytesFreed))
 	}