Add skill command env patching

lib/mk-skill.nix

@@ -3,7 +3,45 @@
   pkgs,
   src,
   name ? builtins.baseNameOf (toString src),
+  commands ? [ ],
 }:
+let
+  lib = pkgs.lib;
+
+  patchEnvDefault = command: envName: value:
+    let
+      marker = "# mkSkill:${envName}";
+      replacement = '': "''${${envName}:=${value}}" ${marker}'';
+    in
+    ''
+      if ! awk -v marker=${lib.escapeShellArg marker} -v replacement=${lib.escapeShellArg replacement} '
+        index($0, marker) { print replacement; found = 1; next }
+        { print }
+        END { if (!found) exit 42 }
+      ' "$_mkskill_command" > "$_mkskill_command.tmp"; then
+        rm -f "$_mkskill_command.tmp"
+        echo "mkSkill: command '${command.path}' has no ${marker} marker" >&2
+        exit 1
+      fi
+      mv "$_mkskill_command.tmp" "$_mkskill_command"
+      chmod +x "$_mkskill_command"
+    '';
+
+  patchCommand = command:
+    let
+      env = command.env or { };
+    in
+    ''
+      _mkskill_command="$out/${command.path}"
+      test -f "$_mkskill_command" \
+        || { echo "mkSkill: command '${command.path}' does not exist" >&2; exit 1; }
+      test -x "$_mkskill_command" \
+        || { echo "mkSkill: command '${command.path}' is not executable" >&2; exit 1; }
+      ${lib.concatStringsSep "\n" (lib.mapAttrsToList (patchEnvDefault command) env)}
+    '';
+
+  patchCommands = lib.concatStringsSep "\n" (map patchCommand commands);
+in
 pkgs.stdenvNoCC.mkDerivation {
   inherit name src;
 
@@ -18,6 +56,7 @@ pkgs.stdenvNoCC.mkDerivation {
     cp -R "$src"/. "$out/"
     test -f "$out/SKILL.md" \
       || { echo "mkSkill: skill '${name}' has no SKILL.md at its root" >&2; exit 1; }
+${patchCommands}
     runHook postInstall
   '';