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/trees/0098-validate-binary-search-tree.org

@@ -1,18 +1,76 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0098. Validate Binary Search Tree :medium:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0098. Validate Binary Search Tree][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0098. Validate Binary Search Tree][0098. Validate Binary Search Tree]]
 :END:
 
+Given the ~root~ of a binary tree, /determine if it is a valid binary search tree (BST)/.
+
+A *valid BST* is defined as follows:
+
+ - The left subtree of a node contains only nodes with keys *strictly less than* the node's key.
+
+ - The right subtree of a node contains only nodes with keys *strictly greater than* the node's key.
+
+ - Both the left and right subtrees must also be binary search trees.
+
+*Example 1:*
+
+
+#+begin_src
+Input: root = [2,1,3]
+Output: true
+#+end_src
+
+
+*Example 2:*
+
+
+#+begin_src
+Input: root = [5,1,4,null,null,3,6]
+Output: false
+Explanation: The root node's value is 5 but its right child's value is 4.
+#+end_src
+
+
+*Constraints:*
+
+ - The number of nodes in the tree is in the range ~[1, 10^{4}]~.
+
+ - ~-2^{31} <= Node.val <= 2^{31} - 1~
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class Solution {
+public:
+    bool isValidBST(TreeNode* root) {
+        
+    }
+};
 #+end_src