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/0268-missing-number.org

@@ -1,18 +1,68 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0268. Missing Number :easy:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0268. Missing Number][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0268. Missing Number][0268. Missing Number]]
 :END:
 
+Given an array ~nums~ containing ~n~ distinct numbers in the range ~[0, n]~, return /the only number in the range that is missing from the array./
+
+*Example 1:*
+
+*Input:* nums = [3,0,1]
+
+*Output:* 2
+
+*Explanation:*
+
+~n = 3~ since there are 3 numbers, so all numbers are in the range ~[0,3]~. 2 is the missing number in the range since it does not appear in ~nums~.
+
+*Example 2:*
+
+*Input:* nums = [0,1]
+
+*Output:* 2
+
+*Explanation:*
+
+~n = 2~ since there are 2 numbers, so all numbers are in the range ~[0,2]~. 2 is the missing number in the range since it does not appear in ~nums~.
+
+*Example 3:*
+
+*Input:* nums = [9,6,4,2,3,5,7,0,1]
+
+*Output:* 8
+
+*Explanation:*
+
+~n = 9~ since there are 9 numbers, so all numbers are in the range ~[0,9]~. 8 is the missing number in the range since it does not appear in ~nums~.
+
+*Constraints:*
+
+ - ~n == nums.length~
+
+ - ~1 <= n <= 10^{4}~
+
+ - ~0 <= nums[i] <= n~
+
+ - All the numbers of ~nums~ are *unique*.
+
+*Follow up:* Could you implement a solution using only ~O(1)~ extra space complexity and ~O(n)~ runtime complexity?
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def missingNumber(self, nums: List[int]) -> int:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    int missingNumber(vector<int>& nums) {
+        
+    }
+};
 #+end_src