add declerative devin conf (#39)

config/devin/config.json

@@ -0,0 +1,9 @@
+{
+  "shell": {
+    "setup_complete": true
+  },
+  "theme_mode": "dark",
+  "agent": {
+    "model": "gpt-5-4-xhigh-priority"
+  }
+}

home/common.nix

@@ -7,6 +7,7 @@
     ./bat.nix
     ./eza.nix
     ./claude.nix
+    ./devin.nix
     ./xdg.nix
     ./security.nix
     ./codex.nix

home/devin.nix

@@ -0,0 +1,26 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+let
+  sourceFile = ../config/devin/config.json;
+  targetDir = "${config.xdg.configHome}/devin";
+  targetFile = "${targetDir}/config.json";
+  bin = "${pkgs.coreutils}/bin";
+in
+{
+  # Devin rewrites this file when settings change, so seed a mutable copy
+  # instead of pointing the path at the read-only Nix store.
+  home.activation.installDevinConfig = lib.hm.dag.entryAfter [ "writeBoundary" ] ''
+    ${bin}/mkdir -p "${targetDir}"
+
+    if [ -f "${targetFile}" ] && ! ${bin}/cmp -s "${sourceFile}" "${targetFile}"; then
+      timestamp="$(${bin}/date +%Y%m%d%H%M%S)"
+      ${bin}/cp "${targetFile}" "${targetFile}.hm-bak.$timestamp"
+    fi
+
+    ${bin}/install -m 600 "${sourceFile}" "${targetFile}"
+  '';
+}