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/0125-valid-palindrome.org

@@ -1,18 +1,65 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0125. Valid Palindrome :easy:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0125. Valid Palindrome][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0125. Valid Palindrome][0125. Valid Palindrome]]
 :END:
 
+A phrase is a *palindrome* if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.
+
+Given a string ~s~, return ~true~/ if it is a *palindrome*, or /~false~/ otherwise/.
+
+*Example 1:*
+
+
+#+begin_src
+Input: s = "A man, a plan, a canal: Panama"
+Output: true
+Explanation: "amanaplanacanalpanama" is a palindrome.
+#+end_src
+
+
+*Example 2:*
+
+
+#+begin_src
+Input: s = "race a car"
+Output: false
+Explanation: "raceacar" is not a palindrome.
+#+end_src
+
+
+*Example 3:*
+
+
+#+begin_src
+Input: s = " "
+Output: true
+Explanation: s is an empty string "" after removing non-alphanumeric characters.
+Since an empty string reads the same forward and backward, it is a palindrome.
+#+end_src
+
+
+*Constraints:*
+
+ - ~1 <= s.length <= 2 * 10^{5}~
+
+ - ~s~ consists only of printable ASCII characters.
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
-
+class Solution:
+    def isPalindrome(self, s: str) -> bool:
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
-
+class Solution {
+public:
+    bool isPalindrome(string s) {
+        
+    }
+};
 #+end_src