Implements issue #121: a script that reads code diffs and flags potential
logic errors including null dereferences, off-by-one patterns, mutable default
arguments, and identity comparisons with literals.
Adds:
- scripts/logic_reviewer.py — core analyzer with AST-based None-deref detection
- scripts/test_logic_reviewer.py — inline test suite (10 tests)
Output: JSON or text report with severity ratings (high/medium/low).
