feat: implement APP_START_CMD for standalone PHP applications

[ramonskie]

Summary

Implements APP_START_CMD support for standalone PHP applications (workers, daemons, CLI apps), restoring full v4.x parity for non-web PHP apps.

Changes

Options Parsing (src/php/options/options.go)

• Parse APP_START_CMD from .bp-config/options.json
• Auto-detect standalone mode when WEB_SERVER is set to "none" and a single .php file exists at app root

Finalize Phase (src/php/finalize/finalize.go)

• Generate Procfile with web: php <APP_START_CMD> when in standalone mode
• Support auto-detection: if only one .php file at root and no web server configured, use it as the start command
• Skip web server and PHP-FPM config when in standalone mode

Integration Tests (src/php/integration/standalone_test.go)

• Explicit APP_START_CMD via options.json
• Auto-detection of single PHP file at root

Fixtures

• fixtures/standalone_app/ — worker app with explicit APP_START_CMD
• fixtures/standalone_autodetect/ — app relying on auto-detection

Usage

{
  "WEB_SERVER": "none",
  "APP_START_CMD": "worker.php"
}

Or simply place a single .php file at the app root with no web server configured.

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Depends on: #1230 (ADDITIONAL_PREPROCESS_CMDS) → #1229 (PHP version placeholders) → #1226 (remove runtime rewrite binary)