fix(cli): preserve existing directories during single-file upload

[shiju-nv]

Summary

Uploading a single file to an existing directory now puts the file inside it, even without a trailing slash. Previously, the upload could report success after replacing an empty directory with the file. Missing destination paths still support intentional filename renames.

Related Issue

Closes #4175

Changes

• Inspect the destination through the authenticated SSH session before choosing the archive entry name.
• Preserve empty and nonempty directories, destination directory symlinks and source symlinks. Keep existing file overwrites, missing-path renames and Git-filtered placement unchanged.
• Recheck the destination type before extraction and inspect it again on transport retries.
• Print the resulting path after an unfiltered upload and explain the placement rules in CLI help and the sandbox docs.
• Add shell-and-tar unit regressions and existing-directory cases to portable file-transfer conformance.

Testing

• Checks appropriate to the affected code and behavior pass
• Unit tests added/updated (if applicable)
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable): sandbox file-transfer docs and CLI help updated