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/0124-binary-tree-maximum-path-sum.org

@@ -1,18 +1,73 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0124. Binary Tree Maximum Path Sum :hard:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0124. Binary Tree Maximum Path Sum][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0124. Binary Tree Maximum Path Sum][0124. Binary Tree Maximum Path Sum]]
 :END:
 
+A *path* in a binary tree is a sequence of nodes where each pair of adjacent nodes in the sequence has an edge connecting them. A node can only appear in the sequence *at most once*. Note that the path does not need to pass through the root.
+
+The *path sum* of a path is the sum of the node's values in the path.
+
+Given the ~root~ of a binary tree, return /the maximum *path sum* of any *non-empty* path/.
+
+*Example 1:*
+
+
+#+begin_src
+Input: root = [1,2,3]
+Output: 6
+Explanation: The optimal path is 2 -> 1 -> 3 with a path sum of 2 + 1 + 3 = 6.
+#+end_src
+
+
+*Example 2:*
+
+
+#+begin_src
+Input: root = [-10,9,20,null,null,15,7]
+Output: 42
+Explanation: The optimal path is 15 -> 20 -> 7 with a path sum of 15 + 20 + 7 = 42.
+#+end_src
+
+
+*Constraints:*
+
+ - The number of nodes in the tree is in the range ~[1, 3 * 10^{4}]~.
+
+ - ~-1000 <= Node.val <= 1000~
+
 ** 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 maxPathSum(self, root: Optional[TreeNode]) -> int:
 #+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:
+    int maxPathSum(TreeNode* root) {
+        
+    }
+};
 #+end_src