[sync] Show diffs for overwritten resources (#66)

[coisa]

Summary

• add a small overwrite diff helper so copy-resource --overwrite can classify replacements as changed, unchanged, binary, or unreadable
• show unified diffs for changed text files, skip no-op overwrites when contents already match, and keep binary/unreadable fallbacks readable in terminal and CI logs
• document the new overwrite reporting in the copy-resource and dev-tools:sync command pages

Testing

• ./vendor/bin/phpunit tests/Console/Command/CopyResourceCommandTest.php tests/Resource/OverwriteDiffRendererTest.php
• ./vendor/bin/phpunit tests/Console/Command tests/Console/CommandLoader tests/Composer/Capability tests/Console/DevToolsTest.php
• git diff --check

Notes

• composer dev-tools was also run, but it still fails in this environment because Rector/PHP CS Fixer parallel workers cannot bind tcp://127.0.0.1:0 (EPERM) and Composer cannot reach Packagist (curl error 6).
• The PHPUnit command suite still emits the existing JoliNotif third-party subscriber warning at shutdown in this environment.

Closes #66

[github-actions]

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• Docs: https://php-fast-forward.github.io/dev-tools/previews/pr-110/
• Coverage: https://php-fast-forward.github.io/dev-tools/previews/pr-110/coverage/
• Metrics: https://php-fast-forward.github.io/dev-tools/previews/pr-110/metrics/