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/two-pointers/0015-3sum.org

@@ -1,18 +1,69 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0015. 3Sum :medium:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0015. 3Sum][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0015. 3Sum][0015. 3Sum]]
 :END:
 
+Given an integer array nums, return all the triplets ~[nums[i], nums[j], nums[k]]~ such that ~i != j~, ~i != k~, and ~j != k~, and ~nums[i] + nums[j] + nums[k] == 0~.
+
+Notice that the solution set must not contain duplicate triplets.
+
+*Example 1:*
+
+
+#+begin_src
+Input: nums = [-1,0,1,2,-1,-4]
+Output: [[-1,-1,2],[-1,0,1]]
+Explanation: 
+nums[0] + nums[1] + nums[2] = (-1) + 0 + 1 = 0.
+nums[1] + nums[2] + nums[4] = 0 + 1 + (-1) = 0.
+nums[0] + nums[3] + nums[4] = (-1) + 2 + (-1) = 0.
+The distinct triplets are [-1,0,1] and [-1,-1,2].
+Notice that the order of the output and the order of the triplets does not matter.
+#+end_src
+
+
+*Example 2:*
+
+
+#+begin_src
+Input: nums = [0,1,1]
+Output: []
+Explanation: The only possible triplet does not sum up to 0.
+#+end_src
+
+
+*Example 3:*
+
+
+#+begin_src
+Input: nums = [0,0,0]
+Output: [[0,0,0]]
+Explanation: The only possible triplet sums up to 0.
+#+end_src
+
+
+*Constraints:*
+
+ - ~3 <= nums.length <= 3000~
+
+ - ~-10^{5} <= nums[i] <= 10^{5}~
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def threeSum(self, nums: list[int]) -> list[list[int]]:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    vector<vector<int>> threeSum(vector<int>& nums) {
+        
+    }
+};
 #+end_src