fix(cli): preserve symlinks during sandbox upload

[mjamiv]

Summary

• build sandbox upload archives with symlink metadata instead of following links
• preserve symlink entries for whole-directory and git-style file-list uploads
• keep existing directory archive shape and file-list missing-entry behavior

Fixes #1425.

Testing

• cargo fmt --all -- --check
• git diff --check
• cargo test -p openshell-cli archive_preserves_symlink
• cargo test -p openshell-cli

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[johntmyers]

[Warning] The archive writer preserves symlinks once entries reach it, but single symlink uploads can still be dereferenced or skipped before that path runs.

The common case is:

openshell sandbox upload <sandbox> ./link

That command still goes through the default git-filtered upload path. git_sync_files canonicalizes the requested path, which follows the symlink. That means:

• If ./link -> tracked-target.txt, the upload can send tracked-target.txt rather than the symlink entry.
• If ./link -> untracked-target.txt or the link is dangling, the upload can produce an empty file list or fail the current Path::exists() guard.
• The create-time upload path has a similar issue for missing/dangling symlinks: the entry can be skipped while the command still reports that files were uploaded.

So the whole-directory/archive implementation looks good, but the single-path entry point still leaves an important symlink-preservation case unfixed.

Suggested fix:

• Detect a single local symlink with symlink_metadata before entering git_sync_files / canonicalization.
• Route that case through sandbox_sync_up directly so the symlink itself reaches write_upload_archive.
• Replace upload existence checks based on Path::exists() with a symlink-aware helper. exists() returns false for dangling symlinks, but a dangling symlink is still a filesystem entry that should either be preserved intentionally or rejected with a clear error.
• Add regression coverage for both a tracked single symlink and a dangling symlink.