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/ch8-back-and-forth.md

@@ -0,0 +1,29 @@
+---
+title: "Back and Forth — XOR"
+event: "GEMASTIK 2026 Warmup"
+challenge: "Back and Forth"
+category: "reverse"
+points: 500
+difficulty: "easy"
+flag: "GEMASTIK19{x0r_it_back_4gain}"
+---
+
+# Back and Forth (500 pts) — Reversing / XOR
+
+**File:** `chall.c`
+
+Flag di-XOR dengan kunci 1-byte `0x42`. Balikkan dengan XOR lagi:
+
+```python
+enc = [0x05, 0x07, 0x0f, 0x03, 0x11, 0x16, 0x0b, 0x09, 0x73, 0x7b, 0x39, 0x3a,
+       0x72, 0x30, 0x1d, 0x2b, 0x36, 0x1d, 0x20, 0x23, 0x21, 0x29, 0x1d, 0x76,
+       0x25, 0x23, 0x2b, 0x2c, 0x3f]
+
+flag = "".join(chr(b ^ 0x42) for b in enc)
+print(flag)
+# GEMASTIK19{x0r_it_back_4gain}
+```
+
+## Flag
+
+`GEMASTIK19{x0r_it_back_4gain}`