feat(desktop): reusable screenshot workflow for agents (#826)

Signed-off-by: Will Pfleger <wpfleger@block.xyz>

scripts/post-screenshots.sh

@@ -0,0 +1,64 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if [[ $# -lt 2 ]]; then
+  echo "Usage: $0 <pr-number> <png-dir> [comment-body-file]" >&2
+  exit 1
+fi
+
+PR="$1"
+PNG_DIR="$2"
+BODY_FILE="${3:-}"
+
+if ! [[ "$PR" =~ ^[0-9]+$ ]]; then
+  echo "error: PR number must be a positive integer" >&2
+  exit 1
+fi
+
+GH_USER=$(gh api user --jq .login)
+BRANCH="agent-screenshots/${GH_USER}"
+REPO="block/sprout"
+RAW_BASE="https://raw.githubusercontent.com/${REPO}/refs/heads/${BRANCH}"
+
+mapfile -t PNGS < <(find "$PNG_DIR" -maxdepth 1 -name "*.png" -type f | sort)
+if [[ ${#PNGS[@]} -eq 0 ]]; then
+  echo "error: no PNGs found in $PNG_DIR" >&2
+  exit 1
+fi
+
+EXISTING_ENTRIES=""
+if git fetch origin "refs/heads/${BRANCH}:refs/remotes/origin/${BRANCH}" 2>/dev/null; then
+  EXISTING_ENTRIES=$(git ls-tree "origin/${BRANCH}" | grep -v $'\t'"\"\\{0,1\\}pr-${PR}--" || true)
+fi
+
+NEW_ENTRIES=""
+IMAGE_URLS=()
+for PNG in "${PNGS[@]}"; do
+  FILENAME=$(basename "$PNG")
+  BLOB=$(git hash-object -w "$PNG")
+  TREE_PATH="pr-${PR}--${FILENAME}"
+  NEW_ENTRIES+="$(printf '100644 blob %s\t%s' "$BLOB" "$TREE_PATH")"$'\n'
+  IMAGE_URLS+=("${RAW_BASE}/${TREE_PATH}")
+done
+
+COMBINED=$(printf '%s\n' "$EXISTING_ENTRIES" "$NEW_ENTRIES" | grep -v '^$')
+TREE=$(echo "$COMBINED" | git mktree)
+
+COMMIT=$(git commit-tree "$TREE" -m "screenshots: PR #${PR}")
+git push --force-with-lease origin "${COMMIT}:refs/heads/${BRANCH}"
+
+IMAGES_SECTION=""
+for URL in "${IMAGE_URLS[@]}"; do
+  FILENAME=$(basename "$URL")
+  NAME="${FILENAME%.png}"
+  IMAGES_SECTION+="![${NAME}](${URL})"$'\n\n'
+done
+
+if [[ -n "$BODY_FILE" ]]; then
+  COMMENT_BODY="$(cat "$BODY_FILE")"$'\n\n'"${IMAGES_SECTION}"
+else
+  COMMENT_BODY="## Screenshots"$'\n\n'"${IMAGES_SECTION}"
+fi
+
+gh pr comment "$PR" --repo "$REPO" --body "$COMMENT_BODY"
+echo "Posted ${#PNGS[@]} screenshot(s) to PR #${PR}"