[TASK] Prioritize composer.json over environment variable for extension key

[CybotTM]

Summary

This PR changes the extension key resolution order to prioritize composer.json over the TYPO3_EXTENSION_KEY environment variable.

New precedence:

1. CLI argument (highest priority)
2. composer.json at [extra][typo3/cms][extension-key] (recommended)
3. TYPO3_EXTENSION_KEY environment variable (deprecated, BC only)

Changes

• CommandHelper.php: Reordered resolution logic, added deprecation warning when using env var
• README.md: Updated documentation and CI examples to reflect new approach
• .env.dist: Removed TYPO3_EXTENSION_KEY (new setups should use composer.json)
• Tests: Updated to verify deprecation warning is triggered

Why?

1. Single source of truth: Extension key belongs in composer.json alongside other extension metadata
2. CI log masking issue: When TYPO3_EXTENSION_KEY is configured as a secret, error messages get masked (e.g. "Could not publish extension ***"), hiding valuable information in error reports

Backwards Compatibility

The environment variable still works for existing setups but triggers a deprecation warning encouraging migration to composer.json.

Resolves: #88