feat: Phase 14 — hierarchical folder structure (GitHub-style nested folders)

- Section index pages ([section]/page.tsx): shows folder tree + flat doc list
- Folder index pages: [section]/[...slug]/page.tsx detects folder paths
  and renders subfolder + document listing instead of 404
- Sidebar tree: hierarchical grouping from flat doc slugs, folder icons (📁)
  with proper indentation per depth level
- DocForm v2: parent folder dropdown (populated from existing folders),
  slug input for leaf name, resolved path display
- Core helpers: extractFoldersForSection + classifyPath exported from @mcpedia/core
- CTF writeup reorganized: pwn-100 ret2win moved into pwn/ subfolder
- _index.md folder intro pages for ctf/ and defcon-quals-2024/
- STANDARD_KEYS includes extraFields to avoid badge duplication

content/writeups/ctf/_index.md

@@ -0,0 +1,26 @@
+---
+id: ctf
+title: "CTF Writeups"
+type: writeup
+tags:
+  - ctf
+  - index
+status: published
+author: asep
+event: "CTF Archive"
+category: index
+difficulty: easy
+points: 0
+created_at: 2026-08-20
+updated_at: 2026-08-20
+---
+
+# CTF Writeups
+
+A collection of CTF challenge writeups organized by event. Each event folder
+contains challenge writeups grouped by category (pwn, crypto, web, forensics, etc.).
+
+## Events
+
+- [DEF CON CTF Quals 2024](defcon-quals-2024) — 1 challenge solved (pwn)
+- [Template](template) — Use as a starting point for new writeups