fix: review -- clean up misleading flake8 comments, remove no-op list comprehension

1. .flake8: removed dead extend-ignore (select already filters). Clarified
   that style codes are intentionally excluded, only bugs checked.
2. dna_extractor.py: removed discarded list comprehension on line 826 that
   checked for test directories but never used the result.

Skipped: removing security job path gate (trufflehog already scans diff
only, Trivy on docs-only PRs wastes CI minutes for zero value).

289 tests pass. Zero flake8 errors.

Author: DevanshuNEU

backend/.flake8

@@ -1,19 +1,13 @@
 [flake8]
 max-line-length = 120
 
-# What we check (F = pyflakes errors, E9 = runtime errors, W6 = deprecated)
+# Only check for real bugs, not style.
+# F = pyflakes (unused imports, undefined names, shadowed vars)
+# E9 = runtime errors (syntax errors, indentation)
+# W6 = deprecated features
+# Style codes (E1/E2/E3/E5/W1/W2/W3/W5) are intentionally excluded.
 select = E9,F,W6
 
-# What we ignore
-# E501: line too long (handled by max-line-length for selected codes)
-# W503/W504: line break before/after binary operator (PEP8 contradicts itself)
-# E128/E127: continuation line indentation (style preference)
-# E226: missing whitespace around operator (style preference)
-# E261: spacing before inline comment (style preference)
-# E301/E302/E303: blank line issues (style preference)
-# E402: module level import not at top (needed for dotenv loading in main.py)
-extend-ignore = E501,W503,W504
-
 # Directories to skip
 exclude =
     .git,

backend/services/dna_extractor.py

@@ -822,9 +822,6 @@ def _extract_test_patterns(self, files: List[Path], repo_path: Path) -> TestPatt
         conftest_files = list(repo_path.rglob('conftest.py'))
         pattern.has_conftest = len(conftest_files) > 0
 
-        # Check for test directory structure
-        [d for d in ['tests', 'test'] if (repo_path / d).exists()]
-        
         for file_path in files:
             try:
                 content = file_path.read_text(encoding='utf-8', errors='ignore')