fix: support overlay copy for directory sync

[laggu]

Summary

• change copy-mode directory sync to overlay source entries into an existing target directory
• make status and unsync operate on the copied source subset for directory copy mode
• document the new overlay semantics in the README, spec, and translations

Testing

• go build ./...
• go test ./...
• go vet ./...

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the directory synchronization behavior in git volume when using copy mode. Previously, syncing a directory in copy mode would overwrite the entire target directory. With this change, sync now performs an 'overlay copy', meaning it merges the source directory's contents into the target, preserving any unrelated files already present in the target. This enhancement also updates status and unsync to manage only the copied subset, providing more granular control and preventing unintended data loss. The changes are reflected across the core logic and comprehensive documentation updates.

Highlights

• Overlay Copy for Directory Sync: The copy mode for directory synchronization now overlays source entries into an existing target directory instead of deleting the entire target. This preserves unrelated files in the target.
• Subset-based Status and Unsync: The status and unsync commands for directory copy mode now operate only on the subset of files that were copied from the source, ignoring other files in the target directory.
• Documentation Updates: The new overlay semantics for directory copy mode have been thoroughly documented in the README, specification, and all available translations.

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[gemini-code-assist]

Code Review

This pull request correctly implements the overlay copy behavior for directory syncs, including updates to sync, unsync, and status logic. The changes are well-tested and the documentation has been updated accordingly across all languages.

My review found a recurring pattern in the new code where errors are returned without being wrapped with additional context. This violates rule #17 of the repository's Go style guide. I've left a couple of comments with suggestions to address this, which will improve debuggability. Overall, this is a solid contribution.

[laggu]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a significant enhancement by adding overlay copy support for directory synchronization. The implementation is robust, including comprehensive updates to documentation across multiple languages and thorough test coverage for the new functionality. I've identified a couple of areas for improvement in the Go code to enhance error handling and security consistency.

[laggu]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces an "overlay-copy" behavior for directories in git-volume's copy mode. This change ensures that when copying directories, sync will overlay source entries into the target, preserving unrelated files, replacing conflicting files/symlinks, and safely failing on file-vs-directory conflicts. The status and unsync commands have been updated to operate on this copied subset. The documentation in README.md and its translations has been updated to reflect these changes. A review comment suggests improving the readability of the new overlay behavior description in the README.md files by converting it into a bulleted list for better clarity.

[gemini-code-assist]

This paragraph contains a lot of important information about the new overlay behavior. To improve readability, consider breaking it down into a bulleted list. This will make it easier for users to quickly understand the different aspects of the copy mode for directories.

For example:

When `mode: "copy"` is used with a directory source, the behavior is as follows:
*   **`sync`**: Overlays source entries into the target directory.
    *   The target root directory is not deleted.
    *   Existing unrelated files are preserved.
    *   File/symlink conflicts are replaced by the source entry.
    *   File-vs-directory conflicts cause the operation to fail safely.
*   **`status` & `unsync`**: Operate only on the set of files copied from the source, ignoring other files in the target directory.

This suggestion should also be applied to the translated README.md files in docs/translations/.