docs: add retro mode docs, bot guide, sprite guide

Update BOT_SETUP.md with full retro mode combo reference, scoring,
and strategy tips. Add docs/bot-guide.md as a standalone developer
guide covering all challenge types including retro mode. Add
docs/sprite-guide.md and sprite-reference.html for custom sprite
creation. Update docs.ts API endpoint and Python example bot with
retro_mode handling.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

examples/python-bot/bot.py

@@ -93,6 +93,16 @@ def handle_challenge(data: dict) -> dict:
     if ctype == "code_golf":
         return {"answer": "print(42)", "trash_talk": "Minimalism."}
 
+    # Retro mode — arcade combo round
+    if ctype == "retro_mode":
+        # Mix known standard moves with a secret super move for discovery bonus
+        combos = [
+            "\u2193\u2192\u2193\u2192+A",  # Hadouken (22 dmg, 1.5x if undiscovered = 33)
+            "\u2192\u2192+A",               # Dash Punch (15 dmg)
+            "\u2191\u2193+A",               # Uppercut (16 dmg)
+        ]
+        return {"answer": " | ".join(combos), "trash_talk": "HADOUKEN!"}
+
     # Factual lookup
     for key, val in FACTS.items():
         if key in lower: