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/0242-valid-anagram.org

@@ -1,18 +1,46 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0242. Valid Anagram :easy:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0242. Valid Anagram][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0242. Valid Anagram][0242. Valid Anagram]]
 :END:
 
+Given two strings ~s~ and ~t~, return ~true~ if ~t~ is an anagram of ~s~, and ~false~ otherwise.
+
+*Example 1:*
+
+*Input:* s = "anagram", t = "nagaram"
+
+*Output:* true
+
+*Example 2:*
+
+*Input:* s = "rat", t = "car"
+
+*Output:* false
+
+*Constraints:*
+
+ - ~1 <= s.length, t.length <= 5 * 10^{4}~
+
+ - ~s~ and ~t~ consist of lowercase English letters.
+
+*Follow up:* What if the inputs contain Unicode characters? How would you adapt your solution to such a case?
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def isAnagram(self, s: str, t: str) -> bool:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    bool isAnagram(string s, string t) {
+        
+    }
+};
 #+end_src