refactor(setup-di-compile) port restored Graycore implementation

[digitalrisedorset]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/mage-os/github-actions/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (not applicable)
• Docs have been added / updated

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Refactoring (no functional changes, no api changes)
• Code style update (formatting, local variables)
• Build related changes
• CI related changes
• Documentation content changes
• Other

What is the current behavior?

setup-di-compile currently embeds several workflow orchestration responsibilities directly inside the action:

• repository checkout
• PHP/composer setup
• changed-files detection
• cache handling
• composer dependency installation

This diverges from the restored Graycore implementation and increases maintenance complexity between forks.

Fixes: N/A

What is the new behavior?

Ports setup-di-compile to match the restored Graycore implementation more closely.

The action is now reduced to a focused compilation step that:

• enables Magento modules
• runs setup:di:compile
• supports configurable Magento root paths through a new path input

Removed from the action:

• embedded actions/checkout
• embedded shivammathur/setup-php
• php_version input
• composer_version input
• tj-actions/changed-files
• conditional execution logic
• cache-magento usage
• embedded composer install

Does this PR introduce a breaking change?

• Yes
• No

Caller workflows must now explicitly:

• checkout the repository
• setup PHP/composer
• install composer dependencies
• prepare Magento before invoking this action

Example:

- uses: actions/checkout@v4

- uses: shivammathur/setup-php@v2
  with:
    php-version: 8.3
    tools: composer:v2
    coverage: none

- run: composer install

- uses: mage-os/github-actions/setup-di-compile@main

Other information

This PR intentionally follows the restored Graycore implementation as closely as possible to reduce divergence between forks and simplify long-term maintenance.

[damienwebdev]

@digitalrisedorset this needs to be labelled a feat!: along with relevant breaking change guidance. LGTM otherwise though.