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/unified_scraper.py

@@ -131,7 +131,7 @@ class UnifiedScraper:
 
         # Write temporary config
         temp_config_path = os.path.join(self.data_dir, 'temp_docs_config.json')
-        with open(temp_config_path, 'w') as f:
+        with open(temp_config_path, 'w', encoding='utf-8') as f:
             json.dump(doc_config, f, indent=2)
 
         # Run doc_scraper as subprocess
@@ -150,7 +150,7 @@ class UnifiedScraper:
         docs_data_file = f"output/{doc_config['name']}_data/summary.json"
 
         if os.path.exists(docs_data_file):
-            with open(docs_data_file, 'r') as f:
+            with open(docs_data_file, 'r', encoding='utf-8') as f:
                 summary = json.load(f)
 
             self.scraped_data['documentation'] = {
@@ -202,7 +202,7 @@ class UnifiedScraper:
 
         # Save data
         github_data_file = os.path.join(self.data_dir, 'github_data.json')
-        with open(github_data_file, 'w') as f:
+        with open(github_data_file, 'w', encoding='utf-8') as f:
             json.dump(github_data, f, indent=2, ensure_ascii=False)
 
         self.scraped_data['github'] = {
@@ -236,7 +236,7 @@ class UnifiedScraper:
 
         # Save data
         pdf_data_file = os.path.join(self.data_dir, 'pdf_data.json')
-        with open(pdf_data_file, 'w') as f:
+        with open(pdf_data_file, 'w', encoding='utf-8') as f:
             json.dump(pdf_data, f, indent=2, ensure_ascii=False)
 
         self.scraped_data['pdf'] = {
@@ -272,10 +272,10 @@ class UnifiedScraper:
             return []
 
         # Load data files
-        with open(docs_data['data_file'], 'r') as f:
+        with open(docs_data['data_file'], 'r', encoding='utf-8') as f:
             docs_json = json.load(f)
 
-        with open(github_data['data_file'], 'r') as f:
+        with open(github_data['data_file'], 'r', encoding='utf-8') as f:
             github_json = json.load(f)
 
         # Detect conflicts
@@ -322,10 +322,10 @@ class UnifiedScraper:
         github_data = self.scraped_data.get('github', {})
 
         # Load data
-        with open(docs_data['data_file'], 'r') as f:
+        with open(docs_data['data_file'], 'r', encoding='utf-8') as f:
             docs_json = json.load(f)
 
-        with open(github_data['data_file'], 'r') as f:
+        with open(github_data['data_file'], 'r', encoding='utf-8') as f:
             github_json = json.load(f)
 
         # Choose merger
@@ -339,7 +339,7 @@ class UnifiedScraper:
 
         # Save merged data
         merged_file = os.path.join(self.data_dir, 'merged_data.json')
-        with open(merged_file, 'w') as f:
+        with open(merged_file, 'w', encoding='utf-8') as f:
             json.dump(merged_data, f, indent=2, ensure_ascii=False)
 
         logger.info(f"✅ Merged data saved: {merged_file}")
@@ -361,7 +361,7 @@ class UnifiedScraper:
         conflicts = []
         conflicts_file = os.path.join(self.data_dir, 'conflicts.json')
         if os.path.exists(conflicts_file):
-            with open(conflicts_file, 'r') as f:
+            with open(conflicts_file, 'r', encoding='utf-8') as f:
                 conflicts_data = json.load(f)
                 conflicts = conflicts_data.get('conflicts', [])