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/graphs/0261-graph-valid-tree.org

@@ -1,7 +1,7 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0261. Graph Valid Tree :medium:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0261. Graph Valid Tree][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0261. Graph Valid Tree][0261. Graph Valid Tree]]
 :END:
 
 ** TODO Approach