fix: Add UTF-8 encoding to all file operations for Windows compatibility

Fixes #209 - UnicodeDecodeError on Windows with non-ASCII characters

**Problem:**
Windows users with non-English locales (Chinese, Japanese, Korean, etc.)
experienced GBK/SHIFT-JIS codec errors when the system default encoding
is not UTF-8.

Error: 'gbk' codec can't decode byte 0xac in position 206: illegal
multibyte sequence

**Root Cause:**
File operations using open() without explicit encoding parameter use
the system default encoding, which on Windows Chinese edition is GBK.
JSON files contain UTF-8 encoded characters that fail to decode with GBK.

**Solution:**
Added encoding='utf-8' to ALL file operations across:
- doc_scraper.py (4 instances):
  * load_config() - line 1310
  * check_existing_data() - line 1416
  * save_checkpoint() - line 173
  * load_checkpoint() - line 186

- github_scraper.py (1 instance):
  * main() config loading - line 922

- unified_scraper.py (10 instances):
  * All JSON read/write operations - lines 134, 153, 205, 239, 275,
    278, 325, 328, 342, 364

**Test Results:**
- ✅ All 612 tests passing (100% pass rate)
- ✅ Backward compatible (UTF-8 is standard on Linux/macOS)
- ✅ Fixes Windows locale issues

**Impact:**
- ✅ Works on ALL Windows locales (Chinese, Japanese, Korean, etc.)
- ✅ Maintains compatibility with Linux/macOS
- ✅ Prevents future encoding issues

**Thanks to:** @my5icol for the detailed bug report and fix suggestion!

src/skill_seekers/cli/doc_scraper.py

@@ -170,7 +170,7 @@ class DocToSkillConverter:
         }
 
         try:
-            with open(self.checkpoint_file, 'w') as f:
+            with open(self.checkpoint_file, 'w', encoding='utf-8') as f:
                 json.dump(checkpoint_data, f, indent=2)
             logger.info("  💾 Checkpoint saved (%d pages)", self.pages_scraped)
         except Exception as e:
@@ -183,7 +183,7 @@ class DocToSkillConverter:
             return
 
         try:
-            with open(self.checkpoint_file, 'r') as f:
+            with open(self.checkpoint_file, 'r', encoding='utf-8') as f:
                 checkpoint_data = json.load(f)
 
             self.visited_urls = set(checkpoint_data["visited_urls"])
@@ -1307,7 +1307,7 @@ def load_config(config_path: str) -> Dict[str, Any]:
         'react'
     """
     try:
-        with open(config_path, 'r') as f:
+        with open(config_path, 'r', encoding='utf-8') as f:
             config = json.load(f)
     except json.JSONDecodeError as e:
         logger.error("❌ Error: Invalid JSON in config file: %s", config_path)
@@ -1413,7 +1413,7 @@ def check_existing_data(name: str) -> Tuple[bool, int]:
     """
     data_dir = f"output/{name}_data"
     if os.path.exists(data_dir) and os.path.exists(f"{data_dir}/summary.json"):
-        with open(f"{data_dir}/summary.json", 'r') as f:
+        with open(f"{data_dir}/summary.json", 'r', encoding='utf-8') as f:
             summary = json.load(f)
         return True, summary.get('total_pages', 0)
     return False, 0