feat: add --contains flag to filter words by required letters

Adds a new --contains argument that only includes words containing all
specified letters, complementing the existing --exclude functionality.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

wordle_helper.py

@@ -22,6 +22,11 @@ def main() -> int:
         metavar="LETTERS",
         help="Exclude words containing any of these letters (case-insensitive)",
     )
+    parser.add_argument(
+        "--contains",
+        metavar="LETTERS",
+        help="Only include words that contain all of these letters (case-insensitive)",
+    )
     parser.add_argument(
         "--mask",
         metavar="PATTERN",
@@ -50,6 +55,7 @@ def main() -> int:
 
     only_set = set(args.only.lower()) if args.only else None
     exclude_set = set(args.exclude.lower()) if args.exclude else None
+    contains_set = set(args.contains.lower()) if args.contains else None
 
     # Prepare matchers
     # Validate and normalize mask
@@ -90,6 +96,8 @@ def main() -> int:
             return False
         if exclude_set is not None and (set(word.lower()) & exclude_set):
             return False
+        if contains_set is not None and not (contains_set <= set(word.lower())):
+            return False
         if mask_positions is not None:
             wl = word.lower()
             for i, ch in enumerate(mask_positions):