refactor(codex-imagegen): extract backend to packages/baoyu-codex-imagegen workspace

Move the codex-imagegen wrapper out of scripts/ into its own workspace
package alongside baoyu-md, baoyu-chrome-cdp, and baoyu-fetch. Drop the
bash shim — src/main.ts now carries a `#\!/usr/bin/env bun` shebang and
serves as the sole entrypoint. Add scripts/sync-codex-imagegen.sh so
skills/baoyu-image-gen/scripts/codex-imagegen/ can be regenerated from
packages/baoyu-codex-imagegen/src/ for skill self-containment. Update
CLAUDE.md, docs/codex-imagegen-backend.md, and the CI workflow paths
accordingly.

scripts/sync-codex-imagegen.sh

@@ -0,0 +1,59 @@
+#!/bin/bash
+# Sync packages/baoyu-codex-imagegen/src/*.ts (excluding tests) into
+# skills/baoyu-image-gen/scripts/codex-imagegen/ so the skill stays
+# self-contained (no `../../../../packages/...` lookups at runtime).
+#
+# Run this whenever packages/baoyu-codex-imagegen/src/ changes,
+# and always before tagging a release.
+
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+SRC_DIR="$REPO_ROOT/packages/baoyu-codex-imagegen/src"
+DST_DIR="$REPO_ROOT/skills/baoyu-image-gen/scripts/codex-imagegen"
+
+if [[ ! -d "$SRC_DIR" ]]; then
+  echo "Error: source dir missing: $SRC_DIR" >&2
+  exit 1
+fi
+
+mkdir -p "$DST_DIR"
+
+changed=0
+for f in cache.ts logger.ts main.ts parser.ts spawn.ts types.ts validator.ts; do
+  src="$SRC_DIR/$f"
+  dst="$DST_DIR/$f"
+  if [[ ! -f "$src" ]]; then
+    echo "Error: missing source file: $src" >&2
+    exit 1
+  fi
+  if [[ ! -f "$dst" ]] || ! cmp -s "$src" "$dst"; then
+    cp "$src" "$dst"
+    echo "synced: $f"
+    changed=$((changed + 1))
+  fi
+done
+
+chmod +x "$DST_DIR/main.ts"
+
+# Drop any stale .ts files in DST that don't exist in SRC.
+for dst in "$DST_DIR"/*.ts; do
+  [[ -e "$dst" ]] || continue
+  name="$(basename "$dst")"
+  case "$name" in
+    *.test.ts) rm -f "$dst"; echo "removed test artifact: $name" ;;
+    *)
+      if [[ ! -f "$SRC_DIR/$name" ]]; then
+        rm -f "$dst"
+        echo "removed stale: $name"
+        changed=$((changed + 1))
+      fi
+      ;;
+  esac
+done
+
+if [[ "$changed" -eq 0 ]]; then
+  echo "codex-imagegen sync: up to date"
+else
+  echo "codex-imagegen sync: $changed file(s) updated"
+fi