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/arrays-hashing/0001-two-sum.org

@@ -1,18 +1,70 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0001. Two Sum :easy:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0001. Two Sum][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0001. Two Sum][0001. Two Sum]]
 :END:
 
+Given an array of integers ~nums~ and an integer ~target~, return /indices of the two numbers such that they add up to ~target~/.
+
+You may assume that each input would have */exactly/ one solution*, and you may not use the /same/ element twice.
+
+You can return the answer in any order.
+
+*Example 1:*
+
+
+#+begin_src
+Input: nums = [2,7,11,15], target = 9
+Output: [0,1]
+Explanation: Because nums[0] + nums[1] == 9, we return [0, 1].
+#+end_src
+
+
+*Example 2:*
+
+
+#+begin_src
+Input: nums = [3,2,4], target = 6
+Output: [1,2]
+#+end_src
+
+
+*Example 3:*
+
+
+#+begin_src
+Input: nums = [3,3], target = 6
+Output: [0,1]
+#+end_src
+
+
+*Constraints:*
+
+ - ~2 <= nums.length <= 10^{4}~
+
+ - ~-10^{9} <= nums[i] <= 10^{9}~
+
+ - ~-10^{9} <= target <= 10^{9}~
+
+ - *Only one valid answer exists.*
+
+*Follow-up: *Can you come up with an algorithm that is less than ~O(n^{2})~ time complexity?
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    vector<int> twoSum(vector<int>& nums, int target) {
+        
+    }
+};
 #+end_src