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/bit-manipulation/0136-single-number.org

@@ -1,18 +1,54 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0136. Single Number :easy:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0136. Single Number][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0136. Single Number][0136. Single Number]]
 :END:
 
+Given a *non-empty* array of integers ~nums~, every element appears /twice/ except for one. Find that single one.
+
+You must implement a solution with a linear runtime complexity and use only constant extra space.
+
+*Example 1:*
+
+*Input:* nums = [2,2,1]
+
+*Output:* 1
+
+*Example 2:*
+
+*Input:* nums = [4,1,2,1,2]
+
+*Output:* 4
+
+*Example 3:*
+
+*Input:* nums = [1]
+
+*Output:* 1
+
+*Constraints:*
+
+ - ~1 <= nums.length <= 3 * 10^{4}~
+
+ - ~-3 * 10^{4} <= nums[i] <= 3 * 10^{4}~
+
+ - Each element in the array appears twice except for one element which appears only once.
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def singleNumber(self, nums: List[int]) -> int:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    int singleNumber(vector<int>& nums) {
+        
+    }
+};
 #+end_src