check-parens: use SBCL reader for 100% accurate paren validation

Replace Python regex-based string/comment stripper with SBCL's actual
reader. For each lisp block, feeds the code to read-from-string in a
loop (reading all forms). Correctly classifies: package errors (not a
paren problem), reader errors (extra/missing closes), EOF (missing
closes Handles all Common Lisp reader edge cases: character literals
(#\( #\) #\;), block comments, string escaping

projects/check-parens/README.org

@@ -39,4 +39,5 @@ Pre-commit hook:
 
 == Dependencies
 
-None (stdlib Python 3).
+Python 3 + SBCL (for the reader-based validation).
+SBCL must be at `/usr/bin/sbcl` (the default path).