fix: correct zip root path and add idempotent build.prop append

- Fix ZIPROOT path from 3 to 4 levels up (../../../../)

- Add path validation before install

- Replace simple cat append with key-based dedup for build.prop (skips existing keys to prevent duplication on re-flash)

- Update AGENTS.md and README.md to reflect the changes

META-INF/com/google/android/update-binary

@@ -33,7 +33,13 @@ abort() {
 # The script lives at:  META-INF/com/google/android/update-binary
 # Zip root is 3 levels up from the script's directory.
 SCRIPT_DIR=$(cd "$(dirname "$0")" && pwd)
-ZIPROOT=$(cd "$SCRIPT_DIR/../../.." && pwd)
+# 脚本位于 META-INF/com/google/android/update-binary，向上4级到 zip 根
+ZIPROOT=$(cd "$SCRIPT_DIR/../../../../" && pwd)
+
+# 验证路径正确
+if [ ! -f "$ZIPROOT/system/build.prop" ]; then
+  abort "Cannot locate zip root (system/build.prop not found)."
+fi
 
 ui_print "========================================"
 ui_print "  CB-C6s-STU-GSI-Fixes"
@@ -68,8 +74,16 @@ fi
 cp /system/build.prop /system/build.prop.bak 2>/dev/null
 chmod 644 /system/build.prop.bak 2>/dev/null
 
-# APPEND — 使用 >> 追加而非覆盖
-cat "$BUILDPROP_SRC" >> /system/build.prop
+# 去重追加：逐 key 检查，仅追加不存在的条目
+while IFS= read -r line || [ -n "$line" ]; do
+  case "$line" in
+    ''|'#'*) continue;;
+  esac
+  key="${line%%=*}"
+  if ! grep -q "^${key}=" /system/build.prop 2>/dev/null; then
+    echo "$line" >> /system/build.prop
+  fi
+done < "$BUILDPROP_SRC"
 chmod 644 /system/build.prop
 
 ui_print "build.prop: entries appended. Backup at build.prop.bak"