ssh: don't KeyError in list_available_nodes on PBS

cluster_soft is lowercased but the command dict is keyed by original case,
and PBS node-listing is unimplemented; return empty early like htcondor.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: alongd

arc/job/ssh.py

@@ -448,9 +448,10 @@ def list_available_nodes(self) -> list:
             list: lines of the node hostnames.
         """
         cluster_soft = servers[self.server]['cluster_soft'].lower()
-        if cluster_soft == 'htcondor':
+        if cluster_soft in ['htcondor', 'pbs']:
+            # Node listing is not implemented for these schedulers; return empty.
             return list()
-        cmd = list_available_nodes_command[cluster_soft]
+        cmd = list_available_nodes_command[servers[self.server]['cluster_soft']]
         stdout = self._send_command_to_server(command=cmd)[0]
         nodes = list()
         if cluster_soft.lower() in ['oge', 'sge']: