feat: remove SwiftNIO backend from FileSystem

[pepicrft]

Summary

• Remove the SwiftNIO dependency from the package manifest and resolve the lockfile without it.
• Collapse FileSystem onto the new default implementation on non-Windows platforms and delete the env-based backend switch.
• Replace the backend-toggle test with coverage for the default file system behavior.

Testing

• swift test

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Handle replacing non-empty directories

replace(_:with:) now routes through File.System.Move.move(..., options: .init(overwrite: true)). In the vendored POSIX move path, overwrite still relies on rename and does not remove an existing destination directory first, so replacing into a non-empty directory fails with ENOTEMPTY/EEXIST instead of actually replacing it. This breaks callers that use replace to refresh directory trees and also diverges from the Windows branch, which removes the destination before copying.

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[chatgpt-codex-connector]

Support cross-device directory moves

move(from:to:options:) now always uses File.System.Move.move on non-Windows. That move implementation only falls back on EXDEV via a copy+delete path that calls File.System.Copy._copyPOSIX, which explicitly rejects directories, so directory moves across filesystems fail instead of succeeding. This can break normal workflows when source and destination are on different mounts (for example temp dirs vs workspace volumes).

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