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)

.gitignore

@@ -4,3 +4,31 @@ dist
 *.local
 .DS_Store
 .vercel
+
+# Secrets
+.env
+.env.*
+!.env.example
+*.pem
+*.key
+*.p12
+secrets.json
+credentials.json
+
+# IDE / OS
+.vscode/
+.idea/
+*.swp
+Thumbs.db
+
+# Logs / cache
+*.log
+.cache/
+.parcel-cache/
+.eslintcache
+
+# Dev artifacts (excluded from repo)
+test-screenshots/
+test-game.py
+preview_*.png
+rocket_preview.png