Improve `cds get --dry-run` by reporting overwrite conflicts

[RonaldHensbergen]

Summary

cds get --dry-run currently shows planned files but does not indicate which files would conflict without --force.

Problem

• Dry-run mode returns before conflict detection.
• Users cannot see overwrite risk during planning.
• Follow-up non-dry run can fail unexpectedly unless --force is added.

Proposed change

• Include conflict detection in dry-run output.
• Clearly separate:
  • files that would be copied,
  • files that would conflict (content differs / path collision).
• Keep dry-run side-effect free (no file writes).

Acceptance criteria

• cds get --dry-run reports conflict count and representative paths.
• Users can determine whether --force is needed before executing.
• Dry-run still performs no writes and does not create a manifest.