feat(php): add --system-php flag to use locally installed PHP

[eddmann]

Summary

Add a --system-php flag that uses the system's installed PHP binary and extensions instead of downloading static PHP builds.

Description

Currently phpx always downloads and uses static PHP binaries from static-php.dev. Some users may prefer to use their existing system PHP installation, either for:

• Using extensions already configured on their system
• Avoiding binary downloads
• Testing against their production PHP environment
• Environments where static binaries aren't available

Proposed Behavior

When --system-php is specified:

1. PHP resolution: Locate PHP via which php or PATH lookup instead of downloading static builds
2. Version checking: Validate the system PHP version satisfies any php = ">=8.2" constraint in the script metadata
3. Extension checking: Verify required extensions are loaded (php -m). If a required extension is missing, fail with a clear error - do not attempt to compile it
4. Execution: Run the script using the system PHP binary

Behavior Differences

Aspect	Default (static PHP)	--system-php
PHP binary	Downloaded from static-php.dev	System php in PATH
Extensions	From static build tiers	Must be pre-installed
Missing extension	Compile from PECL (future)	Error and exit
Version selection	Auto-downloads matching version	Uses whatever is installed

Example Usage

# Use system PHP for a script
phpx run --system-php script.php

# Use system PHP for a tool
phpx tool --system-php phpstan analyse src/

# Fails if extension not available on system
phpx run --system-php script-needing-redis.php
# Error: extension 'redis' not available in system PHP

Implementation Notes

• Check php -v output to parse version for constraint validation
• Check php -m output to list available extensions
• Should work alongside existing php constraint in metadata
• Consider also supporting PHPX_SYSTEM_PHP=1 environment variable
• Could optionally support --system-php=/path/to/php for explicit binary path

Additional Context

• This is useful for CI environments where PHP is already installed
• Allows users to leverage system-managed extensions (apt, brew, etc.)
• Provides an escape hatch when static builds don't meet requirements