Quiet only the stdout UI under --json, not the log level (closes #112)

Per the decision on the issue (option 1), --json no longer implies Quiet. Folding --json into Quiet pinned the stderr log level to WARN, so prune --json gave a machine consumer no record of the local index rows it deleted. The two effects are now split: a JSON field on log.Options drives only the stdout UI-quiet in setupGlobals, keeping the JSON document clean, while the stderr log level follows --verbose/--debug again (diagnostics have gone to stderr since #82). The same coupling is removed for snapshot verify, snapshot remove and remote info; snapshot list was already decoupled.

Model: opus-4-8

internal/cli/prune.go

@@ -41,7 +41,8 @@ work (e.g. after a crashed backup or to reclaim storage).`,
 				LogOptions: log.Options{
 					Verbose: rootFlags.Verbose,
 					Debug:   rootFlags.Debug,
-					Quiet:   rootFlags.Quiet || opts.JSON,
+					Quiet:   rootFlags.Quiet,
+					JSON:    opts.JSON,
 				},
 			}, func(v *vaultik.Vaultik) error {
 				return v.Prune(opts)