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/ch6-sweet-cookie.md

@@ -0,0 +1,29 @@
+---
+title: "Sweet Cookie — Base64 Cookie"
+event: "GEMASTIK 2026 Warmup"
+challenge: "Sweet Cookie"
+category: "web"
+points: 500
+difficulty: "easy"
+flag: "GEMASTIK19{c00k13_b4s3_d3c0d3}"
+---
+
+# Sweet Cookie (500 pts) — Web
+
+**URL:** `http://15.232.89.109:8002/`
+
+Server mengirim cookie `session`. Nilainya cuma di-encode **Base64** (JSON):
+
+```python
+import base64
+
+# Cookie value from Set-Cookie header
+cookie = "eyJ1c2V..."  # nilai session cookie
+decoded = base64.b64decode(cookie)
+print(decoded)
+# {"user": "guest", "flag": "GEMASTIK19{c00k13_b4s3_d3c0d3}"}
+```
+
+## Flag
+
+`GEMASTIK19{c00k13_b4s3_d3c0d3}`