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/stack/0020-valid-parentheses.org

@@ -1,18 +1,70 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0020. Valid Parentheses :easy:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0020. Valid Parentheses][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0020. Valid Parentheses][0020. Valid Parentheses]]
 :END:
 
+Given a string ~s~ containing just the characters ~'('~, ~')'~, ~'{'~, ~'}'~, ~'['~ and ~']'~, determine if the input string is valid.
+
+An input string is valid if:
+
+ - Open brackets must be closed by the same type of brackets.
+
+ - Open brackets must be closed in the correct order.
+
+ - Every close bracket has a corresponding open bracket of the same type.
+
+*Example 1:*
+
+*Input:* s = "()"
+
+*Output:* true
+
+*Example 2:*
+
+*Input:* s = "()[]{}"
+
+*Output:* true
+
+*Example 3:*
+
+*Input:* s = "(]"
+
+*Output:* false
+
+*Example 4:*
+
+*Input:* s = "([])"
+
+*Output:* true
+
+*Example 5:*
+
+*Input:* s = "([)]"
+
+*Output:* false
+
+*Constraints:*
+
+ - ~1 <= s.length <= 10^{4}~
+
+ - ~s~ consists of parentheses only ~'()[]{}'~.
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def isValid(self, s: str) -> bool:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    bool isValid(string s) {
+        
+    }
+};
 #+end_src