olliesrv: route agent paths by FindAgent, not active agent

The 9P server path classifier still validated /session/{id}/agent/{aid}
against sess.Core.Agent().Name(), which leaked single-agent semantics into
multi-agent routing. This broke per-agent paths even when the per-agent
file trees were otherwise correct.

Fix route/type validation to resolve the requested agent with FindAgent(),
and use that specific agent for /proc PID checks.

cmd/olliesrv/server.go

@@ -585,9 +585,9 @@ func (s *Server) pathType(path string) string {
 			if sess.Core == nil {
 				return ""
 			}
-			// Validate agent Name
-			agentName := sess.Core.Agent().Name()
-			if agentName == "" || parts[2] != agentName {
+			// Validate target agent by name or ID.
+			ag := sess.Core.FindAgent(parts[2])
+			if ag == nil {
 				return ""
 			}
 			// /session/{id}/agent/{aid} — directory
@@ -600,7 +600,7 @@ func (s *Server) pathType(path string) string {
 			}
 			// /session/{id}/agent/{aid}/proc/{pid} — verify exists
 			if len(parts) == 5 && parts[3] == "proc" {
-				for _, p := range sess.Core.Agent().ListDetached() {
+				for _, p := range ag.ListDetached() {
 					if fmt.Sprintf("%d", p.PID) == parts[4] {
 						return "file"
 					}