chore: remove .env.production from version control

- .env.production → .env.production.example (template)
- .gitignore: add .env.production
- git rm --cached .env.production (keep on disk)

Environment-specific configs should not be tracked in git.

.gitignore

@@ -23,6 +23,7 @@ Thumbs.db
 *.local
 .env.local
 .env.*.local
+.env.production
 
 # Test artifacts
 .playwright-mcp/