[filesystem] Isolate Finder creation behind a factory (#90)

[coisa]

Summary

• Add FinderFactoryInterface and FinderFactory so every mutable Symfony Finder scan starts from a fresh instance.
• Replace injected shared Finder usage in resource copy, git hook sync, command loading, and skill synchronization.
• Register the factory in the service provider and remove the direct Finder::class service mapping.
• Move services that consumed Symfony\Component\Filesystem\Filesystem directly to the local FilesystemInterface abstraction.
• Extend the local filesystem wrapper with remove, symlink, and readlink so skill synchronization and other services do not depend on Symfony Filesystem directly.
• Update tests to mock local factories/interfaces and add coverage proving fresh Finder instances and local filesystem link operations.

Verification

• git diff --check
• rg -n -F "Symfony\\Component\\Filesystem\\Filesystem" src tests (only src/Filesystem/Filesystem.php remains as the adapter around Symfony)
• rg -n "Finder \\$|Finder \\$finder|Finder::class|Finder::create|new Finder\\(" src tests
• ./vendor/bin/phpunit --filter 'FilesystemTest|FinderFactoryTest|CopyResourceCommandTest|GitHooksCommandTest|DevToolsCommandLoaderTest|SkillsSynchronizerTest|GitIgnore\\WriterTest|GitAttributes\\WriterTest|ExistenceCheckerTest|GeneratorTest|DevToolsTest' (blocked: env: php: No such file or directory)
• ./vendor/bin/composer dev-tools tests -- --filter='FinderFactoryTest|CopyResourceCommandTest|GitHooksCommandTest|DevToolsCommandLoaderTest|SkillsSynchronizerTest|DevToolsTest' (blocked: env: php: No such file or directory)

Notes

• No production Finder::create() calls were present. The only remaining production new Finder() is inside the dedicated FinderFactory.
• Direct Symfony Filesystem usage now remains only inside the local filesystem adapter.

Closes #90

[github-actions]

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