Scope the PID lock to mutating commands (closes #150)

RunWithApp took the process-wide PID lock for every fx-backed command,
so read-only commands (info, snapshot list, snapshot verify, remote
info) failed with "already running" while a backup held it.

AppOptions now carries a lockMode, and each command declares its mode at
the call site. Only mutating commands (snapshot create, snapshot purge,
snapshot remove, prune, remote nuke) acquire the lock; read-only ones run
without it and are never blocked. snapshot restore is classified
read-only: it writes only to its target directory, not the local index
or remote store. The acquire/skip decision moves to a small
acquireLockIfMutating helper, covered by a test that a read-only command
runs while the lock is held and two mutators still exclude. The README
locking section is rewritten to match.

Model: opus-4-8

internal/cli/prune.go

@@ -44,6 +44,7 @@ work (e.g. after a crashed backup or to reclaim storage).`,
 					Quiet:   rootFlags.Quiet,
 					JSON:    opts.JSON,
 				},
+				Mode: mutating,
 			}, func(v *vaultik.Vaultik) error {
 				return v.Prune(opts)
 			}, func(err error) {