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/0231-power-of-two.org

@@ -1,18 +1,63 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0231. Power of Two :easy:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0231. Power of Two][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0231. Power of Two][0231. Power of Two]]
 :END:
 
+Given an integer ~n~, return /~true~ if it is a power of two. Otherwise, return ~false~/.
+
+An integer ~n~ is a power of two, if there exists an integer ~x~ such that ~n == 2^{x}~.
+
+*Example 1:*
+
+
+#+begin_src
+Input: n = 1
+Output: true
+Explanation: 20 = 1
+#+end_src
+
+
+*Example 2:*
+
+
+#+begin_src
+Input: n = 16
+Output: true
+Explanation: 24 = 16
+#+end_src
+
+
+*Example 3:*
+
+
+#+begin_src
+Input: n = 3
+Output: false
+#+end_src
+
+
+*Constraints:*
+
+ - ~-2^{31} <= n <= 2^{31} - 1~
+
+*Follow up:* Could you solve it without loops/recursion?
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def isPowerOfTwo(self, n: int) -> bool:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    bool isPowerOfTwo(int n) {
+        
+    }
+};
 #+end_src