Allow selecting the environment via CI_ENV

[mezinster]

index.php hardcodes define('ENVIRONMENT', 'development');, and CodeIgniter's standard CI_ENV lookup sits commented out above it. To run in production today, admins have to edit index.php, which then conflicts on every git pull.

This switches to the standard lookup:

define('ENVIRONMENT', isset($_SERVER['CI_ENV']) ? $_SERVER['CI_ENV'] : 'development');

The default is still development, so unlike #2733 this doesn't change behaviour for any existing install and needs no action from users. Admins who want production mode can opt in without touching tracked files:

• Apache: SetEnv CI_ENV production (vhost or .htaccess)
• nginx + PHP-FPM: fastcgi_param CI_ENV production;
• CLI (php index.php migrate): CI_ENV=production php index.php ...

Testing

• php -l index.php on PHP 7.4.33 and 8.2.34
• Apache + PHP 8.2 deployment with SetEnv CI_ENV production: pages render without PHP notices and without the "Developer mode" badge
• Without CI_ENV set, the expression falls back to development, the same value as the current hardcoded line

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused change preserves existing behavior while enabling standard environment configuration.

Review effort: Balanced
Findings: None

What changed in this PR

Enables deployment-specific CodeIgniter environments without editing tracked files.

Changes:

• Reads CI_ENV from $_SERVER.
• Preserves development as the default.

File	Description
index.php	Configures ENVIRONMENT from CI_ENV with the existing fallback.

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