refactor: move DSA notes to org/study_deck_02/

Restructure NeetCode study materials into a dedicated Anki-exportable
deck directory. Separates DSA problem notes from C++ flashcards.

- Move roadmap.org from leetcode/out/ to org/study_deck_02/
- Move 200 note files from org/cpp/dsa/ to org/study_deck_02/dsa/
- Add #+PROPERTY: STUDY_DECK_02 to all org files for org-anki export
- Simplify roadmap format: properties drawer replaces nested headings
- Update NEETCODE backlinks in all note files
- Update extract.mjs and scaffold-notes.mjs for new paths
- Create org/study_deck_02/AGENTS.md with study workflow docs
- Update root and leetcode AGENTS.md

leetcode/out/roadmap-neetcode150.json

@@ -1,6 +1,6 @@
 {
   "source": "https://neetcode.io/roadmap",
-  "extracted": "2026-05-31",
+  "extracted": "2026-06-01",
   "graph": {
     "nodes": [
       {

leetcode/out/roadmap.json

@@ -1,6 +1,6 @@
 {
   "source": "https://neetcode.io/roadmap",
-  "extracted": "2026-05-31",
+  "extracted": "2026-06-01",
   "graph": {
     "nodes": [
       {