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>

server/src/routes/docs.ts

@@ -121,6 +121,17 @@ docsRouter.get('/webhook', (c) => {
         { type: 'code_golf', label: 'Code Golf', timeout_ms: 20000, description: 'Write the shortest code possible.' },
         { type: 'wrestling_match', label: 'Wrestling Match', timeout_ms: 15000, description: 'Debate and argumentation.' },
       ],
+      special: [
+        {
+          type: 'retro_mode',
+          label: 'Retro Mode',
+          timeout_ms: 12000,
+          description: 'Arcade combo round. Submit 3 gamepad combos separated by |. Use ↑↓←→ A B buttons. Longer combos = more damage. Secret combos exist — experiment!',
+          response_format: 'combo1 | combo2 | combo3',
+          example_response: '↓→↓→+A | →→+A | ↑↓+A',
+          note: 'One retro round per fight, randomly placed between rounds 3-8.',
+        },
+      ],
     },
 
     testing: {