feature(gpg): move git from gpg-signing to ssh-signing

modules/home-manager/git.nix

@@ -1,6 +1,17 @@
 { config, pkgs, inputs, ... }:
 
+let
+  # public half of ~/.ssh/id_ed25519, registered and verified as a signing key
+  # in gitea
+  signingKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIHRhwzo1oxaT3fEySSmILKNnu9v30cfjx5G7FKpmfqeX aaron@argon";
+in
 {
+  # gitea verifies signatures against the account key, this teaches the local
+  # git the same trust so `git log --show-signature` resolves as well
+  xdg.configFile."git/allowed_signers".text = ''
+    aaron@0x29a.ch ${signingKey}
+  '';
+
   programs.git = {
     enable = true;
     settings = {
@@ -9,9 +20,14 @@
         name  = "aaron";
         email = "aaron@0x29a.ch";
       };
+      gpg.ssh.allowedSignersFile = "${config.xdg.configHome}/git/allowed_signers";
     };
     signing = {
-      key = "7A830180A05DAC59CDE43B0677D2F5DB48184456";
+      format = "ssh";
+      # point at the *public* key so ssh-keygen signs through the ssh agent
+      # instead of reading the passphrase protected private key from disk
+      key = "${config.home.homeDirectory}/.ssh/id_ed25519.pub";
+      signer = "${pkgs.openssh}/bin/ssh-keygen";
       signByDefault = true;
     };
   };

modules/nixos/gnupg.nix

@@ -4,12 +4,28 @@
   # enable gnupg agent
   programs.gnupg.agent = {
     enable = true;
-    enableSSHSupport = true;
+    # a graphical pinentry never grabs the tty, so a passphrase prompt can no
-    pinentryPackage = pkgs.pinentry-curses;
+    # longer take over a terminal that a coding agent is driving
+    pinentryPackage = pkgs.pinentry-qt;
+    settings = {
+      # keep the key unlocked for a full working day instead of 10 minutes,
+      # so signing commits asks at most once per session
+      default-cache-ttl = 86400;
+      max-cache-ttl = 86400;
+      # fail an unattended signature instead of blocking on a prompt forever
+      pinentry-timeout = 120;
+    };
   };
 
-  environment.systemPackages = with pkgs; [ 
+  environment.systemPackages = with pkgs; [
     gnupg
+    pinentry-qt
+    # fallback for sessions without a display, switch pinentryPackage to use it
     pinentry-curses
+    # prime the passphrase cache on demand, e.g. before an agent session
+    (writeShellScriptBin "gpg-unlock" ''
+      echo | ${config.programs.gnupg.package}/bin/gpg --clearsign --output /dev/null
+      echo "gpg key unlocked"
+    '')
   ];
 }