feat(acp/handlers): fs/read_text_file + fs/write_text_file with path safety

internal/acp/handlers/handlers.go

@@ -38,9 +38,16 @@ type FsError struct {
 	Message string
 }
 
-// FsHandler is the placeholder for fs/read_text_file + fs/write_text_file.
-// Fields and methods are added in Task D4.
-type FsHandler struct{}
+// FsHandler combines fs/* method handlers so Relay can hold one reference.
+type FsHandler struct {
+	Read  *FsReadHandler
+	Write *FsWriteHandler
+}
+
+// NewFsHandler constructs a composite FsHandler with default sub-handlers.
+func NewFsHandler() *FsHandler {
+	return &FsHandler{Read: NewFsReadHandler(), Write: NewFsWriteHandler()}
+}
 
 // PermissionHandler is the placeholder for session/request_permission.
 // Fields and methods are added in Task D5.