fs: set agent dir UID to agent ID, GID to 'agent'

Agent directories now have proper ownership: UID is the agent's
immutable 12-char hex identity, GID is 'agent'. Consistent with
session directories which use UID(s.Uname()).

fs/agentfiles.go

@@ -32,6 +32,8 @@ func listAgents(ctx HandlerCtx) ([]FsNodeDecl, error) {
 		dirName := a.Name()
 
 		agentDir := Dir(dirName,
+			UID(a.ID()),
+			GID("agent"),
 			Alias(a.ID()),
 			Leaf("prompt", 0666, Write(func(_ HandlerCtx, data []byte) error {
 				input := strings.TrimSpace(string(data))