docs: replace dummy docs with GEMASTIK 2026 Warmup writeups (ch1-14)

- Deleted all previous dummy docs (defcon-quals-2024, infra/cloudflare-525, debugging/websocket-timeout, template, docs/*, notes/*, research/*)
- Added 14 Gemastik warmup writeups covering encoding, stego, web, pwn, crypto
- ch13: RSA small factors; ch14: RSA special integers via paper-search (GCD with paper appendix primes)
- All flags recovered and documented with solution methods

content/writeups/gemastik-2026-warmup/ch11-behind-the-picture.md

@@ -0,0 +1,24 @@
+---
+title: "Behind the Picture — PNG Strings"
+event: "GEMASTIK 2026 Warmup"
+challenge: "Behind the Picture"
+category: "forensics"
+points: 500
+difficulty: "easy"
+flag: "GEMASTIK19{c4t_g4mbar_ada_flag}"
+---
+
+# Behind the Picture (500 pts) — Steganografi / PNG
+
+**File:** `kucing.png` (cat image)
+
+Flag disisipkan sebagai string di dalam file PNG (bisa dilihat dengan `strings`):
+
+```bash
+strings kucing.png | grep GEMASTIK
+# GEMASTIK19{c4t_g4mbar_ada_flag}
+```
+
+## Flag
+
+`GEMASTIK19{c4t_g4mbar_ada_flag}`