fix infinite recursion in session/{sid}/agent directory

pathType() only recognized 'proc' as a subdirectory under sessions.
The 'agent' hierarchy (agent/, agent/{aid}/, agent/{aid}/proc/) was
falling through to 'return "file"', causing walk() to succeed with
QTFile at any depth. 9p ls then printed the basename 'agent' forever.

Fix: increase SplitN limit to 5, add directory cases for the full
agent path hierarchy, and verify the session exists before allowing
descent into any sub-path.

server/server.go

@@ -527,19 +527,28 @@ func (s *Server) pathType(path string) string {
 			if isSessionFile(path) {
 				return "file"
 			}
+			// Verify session exists.
+			if session.Lookup(s.sessionTree, name) == nil {
+				return ""
+			}
 			return "dir"
 		}
+		parts := strings.SplitN(name, "/", 5)
+		// Verify session exists before descending.
+		sess := session.Lookup(s.sessionTree, parts[0])
+		if sess == nil {
+			return ""
+		}
 		// /session/{id}/proc — directory
-		parts := strings.SplitN(name, "/", 3)
 		if len(parts) == 2 && parts[1] == "proc" {
 			return "dir"
 		}
+		// /session/{id}/agent — directory
+		if len(parts) == 2 && parts[1] == "agent" {
+			return "dir"
+		}
 		// /session/{id}/proc/{pid} — verify PID exists in detached list
 		if len(parts) == 3 && parts[1] == "proc" {
-			sess := session.Lookup(s.sessionTree, parts[0])
-			if sess == nil {
-				return ""
-			}
 			for _, p := range sess.Core.Agent().ListDetached() {
 				if fmt.Sprintf("%d", p.PID) == parts[2] {
 					return "file"
@@ -547,6 +556,27 @@ func (s *Server) pathType(path string) string {
 			}
 			return ""
 		}
+		// /session/{id}/agent/{aid} — directory
+		if len(parts) == 3 && parts[1] == "agent" {
+			return "dir"
+		}
+		// /session/{id}/agent/{aid}/proc — directory
+		if len(parts) == 4 && parts[1] == "agent" && parts[3] == "proc" {
+			return "dir"
+		}
+		// /session/{id}/agent/{aid}/proc/{pid} — verify exists
+		if len(parts) == 5 && parts[1] == "agent" && parts[3] == "proc" {
+			for _, p := range sess.Core.Agent().ListDetached() {
+				if fmt.Sprintf("%d", p.PID) == parts[4] {
+					return "file"
+				}
+			}
+			return ""
+		}
+		// /session/{id}/agent/{aid}/{file} — file
+		if len(parts) >= 4 && parts[1] == "agent" {
+			return "file"
+		}
 		// /session/{id}/{file} — file
 		return "file"
 	}