treewide: use TODO-EXTERNAL

Use TODO-EXTERNAL for TODOs that depend on external factors like
upstream fixes.

modules/netns-isolation.nix

@@ -70,7 +70,8 @@ let
   #   and
   #   availableNetns.clighting = [ "bitcoind" ];
   #
-  # FIXME: Although negligible for our purposes, this calculation's runtime
+  # TODO-EXTERNAL:
+  # Although negligible for our purposes, this calculation's runtime
   # is in the order of (number of connections * number of services),
   # because attrsets and lists are fully copied on each update with '//' or '++'.
   # This can only be improved with an update in the nix language.

modules/nix-bitcoin.nix

@@ -35,7 +35,7 @@ with lib;
       runAsUserCmd = mkOption {
         readOnly = true;
         default = if config.security.doas.enable
-                  # TODO: Use absolute path until https://github.com/NixOS/nixpkgs/pull/133622 is available.
+                  # TODO-EXTERNAL: Use absolute path until https://github.com/NixOS/nixpkgs/pull/133622 is available.
                   then "/run/wrappers/bin/doas -u"
                   else "sudo -u";
       };

modules/presets/enable-tor.nix

@@ -23,6 +23,7 @@ in {
     lnd = defaultEnableTorProxy;
     lightning-loop = defaultEnableTorProxy;
     liquidd = defaultEnableTorProxy;
+    # TODO-EXTERNAL:
     # disable Tor enforcement until btcpayserver can fetch rates over Tor
     # btcpayserver = defaultEnableTorProxy;
     spark-wallet = defaultEnableTorProxy;

pkgs/python-packages/pyln-bolt7/default.nix

@@ -13,7 +13,7 @@ buildPythonPackage rec {
 
   postUnpack = "sourceRoot=$sourceRoot/contrib/pyln-spec/bolt7";
 
-  # TODO-EXTERNAL
+  # TODO-EXTERNAL:
   # Remove when this fix is released
   # https://github.com/ElementsProject/lightning/pull/4910
   postPatch = ''