Harden ignores and remove dev artifacts

- Expand .gitignore with secret/IDE/log patterns
- Add .vercelignore to keep dev artifacts out of future deploys
- Remove test-game.py, test-screenshots/, preview_*.png, rocket_preview.png
- Update AGENTS.md remote (stale GitHub link -> Gitea)

.vercelignore

@@ -0,0 +1,24 @@
+# Dev artifacts — not needed in Vercel deployment
+test-game.py
+test-screenshots/
+preview_*.png
+rocket_preview.png
+
+# Docs / agent notes
+AGENTS.md
+
+# Source control
+.git/
+.gitignore
+.gitattributes
+
+# Local env / IDE
+.env
+.env.*
+.vscode/
+.idea/
+*.log
+
+# Build artifacts that Vercel regenerates
+node_modules/
+.vite/