feat: populate note files with problem descriptions and code stubs

Add populate-notes.mjs that fetches problem descriptions and
Python/C++ code stubs from LeetCode's GraphQL API. Populated
all 197 NeetCode 150 note files with:
- Problem description (examples, constraints)
- Python code stub (function signature)
- C++ code stub (function signature + includes)

API responses cached in leetcode/.cache/leetcode/ for instant re-runs.

org/study_deck_02/dsa/greedy/0678-valid-parenthesis-string.org

@@ -1,18 +1,65 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0678. Valid Parenthesis String :medium:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0678. Valid Parenthesis String][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0678. Valid Parenthesis String][0678. Valid Parenthesis String]]
 :END:
 
+Given a string ~s~ containing only three types of characters: ~'('~, ~')'~ and ~'*'~, return ~true~ /if/ ~s~ /is *valid*/.
+
+The following rules define a *valid* string:
+
+ - Any left parenthesis ~'('~ must have a corresponding right parenthesis ~')'~.
+
+ - Any right parenthesis ~')'~ must have a corresponding left parenthesis ~'('~.
+
+ - Left parenthesis ~'('~ must go before the corresponding right parenthesis ~')'~.
+
+ - ~'*'~ could be treated as a single right parenthesis ~')'~ or a single left parenthesis ~'('~ or an empty string ~""~.
+
+*Example 1:*
+
+
+#+begin_src
+Input: s = "()"
+Output: true
+#+end_src
+*Example 2:*
+
+
+#+begin_src
+Input: s = "(*)"
+Output: true
+#+end_src
+*Example 3:*
+
+
+#+begin_src
+Input: s = "(*))"
+Output: true
+#+end_src
+
+
+*Constraints:*
+
+ - ~1 <= s.length <= 100~
+
+ - ~s[i]~ is ~'('~, ~')'~ or ~'*'~.
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def checkValidString(self, s: str) -> bool:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    bool checkValidString(string s) {
+        
+    }
+};
 #+end_src