feat(builder): support project-local distribution scripts

skills/builder/scripts/copy_artifact.sh

@@ -6,12 +6,17 @@ usage() {
 Usage:
   COPY_DEST=/path/or/container:/dir \
     copy_artifact.sh FILE [FILE ...]
+  COPY_SCRIPT=.pouch/builder/publish-apk.sh \
+    copy_artifact.sh FILE [FILE ...]
 
-Copy built artifacts to COPY_DEST. Dest is a local directory, or a Docker
-copy target `container:/absolute/dir` (docker cp). Does not read `.env`.
+Copy built artifacts using COPY_SCRIPT when configured; otherwise copy to
+COPY_DEST. COPY_DEST is a local directory or a Docker copy target
+`container:/absolute/dir` (docker cp). Does not read `.env`.
 
 Environment (project root .env.builder; shell values win):
-  COPY_DEST              Required. Local dir or container:/dir
+  COPY_DEST              Optional fallback. Local dir or container:/dir
+  COPY_SCRIPT            Optional .pouch/builder/ relative path or absolute
+                         executable. Receives FILE paths as positional args.
   COPY_AS                Optional. Extra basenames for the first file
                          (comma or space separated)
   ALLOW_UNCOMMITTED=1    Publish despite a dirty working tree
@@ -58,9 +63,13 @@ while (($#)); do
   esac
 done
 
-if [[ -z "$dest" || $# -eq 0 ]]; then
-  echo "Error: COPY_DEST and at least one file are required." >&2
-  echo "Set them in the environment or the project root .env.builder." >&2
+if [[ -z "$dest" && -z "${COPY_SCRIPT:-}" || $# -eq 0 ]]; then
+  if [[ $# -eq 0 ]]; then
+    echo "Error: at least one file is required." >&2
+  else
+    echo "Error: set COPY_DEST or COPY_SCRIPT." >&2
+  fi
+  echo "Set the value in the environment or the project root .env.builder." >&2
   usage >&2
   exit 2
 fi
@@ -73,6 +82,63 @@ if [[ -n "$project_root" ]] && git -C "$project_root" rev-parse HEAD >/dev/null
   fi
 fi
 
+if [[ -n "${COPY_SCRIPT:-}" ]]; then
+  copy_script=$COPY_SCRIPT
+  if [[ "$copy_script" != /* ]]; then
+    relative_script=${copy_script#./}
+    if [[ "$relative_script" != .pouch/builder/* || -z "$project_root" ]]; then
+      echo "Error: relative COPY_SCRIPT must be under .pouch/builder/." >&2
+      exit 2
+    fi
+    copy_script="$project_root/$relative_script"
+  fi
+  if [[ ! -f "$copy_script" ]]; then
+    echo "Error: COPY_SCRIPT is not a file: $copy_script" >&2
+    exit 2
+  fi
+  if [[ ! -x "$copy_script" ]]; then
+    echo "Error: COPY_SCRIPT is not executable: $copy_script" >&2
+    exit 2
+  fi
+  artifact_args=()
+  for src in "$@"; do
+    if [[ ! -f "$src" ]]; then
+      echo "Error: not a file: $src" >&2
+      exit 2
+    fi
+    if [[ "$src" == /* ]]; then
+      artifact_args+=("$src")
+    else
+      artifact_args+=("$PWD/$src")
+    fi
+  done
+  if $dry_run; then
+    printf 'custom copy script:'
+    printf ' %q' "$copy_script" "${artifact_args[@]}"
+    printf '\n'
+    exit 0
+  fi
+  export BUILDER_PROJECT_ROOT=${project_root:-}
+  if "$copy_script" "${artifact_args[@]}"; then
+    if command -v sha256sum >/dev/null 2>&1; then
+      for src in "${artifact_args[@]}"; do
+        sha256sum -- "$src"
+      done
+    fi
+    echo "copy_artifact: custom script completed for $# file(s)"
+    exit 0
+  else
+    status=$?
+    echo "Error: COPY_SCRIPT failed with exit code $status." >&2
+    exit "$status"
+  fi
+fi
+
+if [[ -z "$dest" ]]; then
+  echo "Error: COPY_DEST is required when COPY_SCRIPT is not configured." >&2
+  exit 2
+fi
+
 is_docker_dest() {
   local d=$1
   [[ "$d" != /* && "$d" != ./* && "$d" != ../* && "$d" != ~* ]] \