feat(import): Add --no-merge flag for consistency with add/discover/fmt

[tony]

Summary

The add, discover, and fmt subcommands all support a --no-merge flag that skips merging duplicate workspace root sections before writing. The import subcommand currently lacks this flag, which is inconsistent.

Context

Identified during multi-model code review of PR #520 (finding #7). The _run_import() function in src/vcspull/cli/import_cmd/_common.py now uses DuplicateAwareConfigReader and merge_duplicate_workspace_roots() (added in PR #520), but does not expose a --no-merge option to skip the merge step.

Expected Behavior

$ vcspull import gh my-org \
    --workspace ~/code/ \
    --no-merge

When --no-merge is passed, import should preserve duplicate workspace root sections as-is in the config file, matching the behavior of vcspull add --no-merge.

Implementation Notes

• Add --no-merge to _create_shared_parent() in _common.py
• Thread the flag through _run_import() to conditionally skip merge_duplicate_workspace_roots()
• Follow the same pattern as add.py's _prepare_no_merge_items() for the no-merge save path
• Use save_config_yaml_with_items() for YAML to preserve duplicate sections