fix: Resolve 21 ruff linting errors (SIM102, SIM117, B904, SIM113, B007)

Fixed all 21 linting errors identified in GitHub Actions:

SIM102 (7 errors - nested if statements):
- config_extractor.py:468 - Combined nested conditions
- config_validator.py (was B904, already fixed)
- pattern_recognizer.py:430,538,916 - Combined nested conditions
- test_example_extractor.py:365,412,460 - Combined nested conditions
- unified_skill_builder.py:1070 - Combined nested conditions

SIM117 (9 errors - multiple with statements):
- test_install_agent.py:418 - Combined with statements
- test_issue_219_e2e.py:278 - Combined with statements
- test_llms_txt_downloader.py:33,88 - Combined with statements
- test_skip_llms_txt.py:75,98,121,148,172,304 - Combined with statements

B904 (1 error - exception handling):
- config_validator.py:62 - Added 'from e' to exception chain

SIM113 (1 error - enumerate usage):
- doc_scraper.py:1068 - Removed unused 'completed' counter variable

B007 (1 error - unused loop variable):
- pdf_scraper.py:167 - Changed 'keywords' to '_' for unused variable

All changes improve code quality without altering functionality.
Tests: 1214 passed, 167 skipped (4 pre-existing failures unrelated)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/skill_seekers/cli/doc_scraper.py

@@ -1056,7 +1056,6 @@ class DocToSkillConverter:
                             futures.append(future)
 
                     # Wait for some to complete before submitting more
-                    completed = 0
                     for future in as_completed(futures[:batch_size]):
                         # Check for exceptions
                         try:
@@ -1065,8 +1064,6 @@ class DocToSkillConverter:
                             with self.lock:
                                 logger.warning("  ⚠️  Worker exception: %s", e)
 
-                        completed += 1
-
                         with self.lock:
                             self.pages_scraped += 1