Fix selected public workspace prompt migration

[paullizer]

Summary

• Copy selected public workspace prompts using current public_id ownership while retaining legacy public_workspace_id compatibility.
• Deduplicate transitional prompts containing both ownership fields and apply the same scope logic to reconciliation.
• Add focused coverage for selected, unselected, transitional, artifact-count, and all-workspace behavior.

Validation

• .venv\Scripts\python.exe -m pytest -q functional_tests/test_data_management_public_prompt_migration.py functional_tests/test_data_management_cosmos_migration_resilience.py functional_tests/test_data_management_migration_reconciliation.py functional_tests/test_data_management_migration_orchestration.py functional_tests/test_data_management_security_patterns.py functional_tests/test_file_sync_azure_blob_storage.py functional_tests/test_file_sync_azure_files_identity.py functional_tests/test_file_sync_onedrive_personal.py (59 passed)
• .venv\Scripts\python.exe -m pytest -q functional_tests/test_data_management_migration_preflight.py functional_tests/test_data_management_migration_state.py functional_tests/test_data_management_incremental_migration_modes.py functional_tests/test_data_management_migration_manifest.py functional_tests/test_data_management_migration_provenance.py (12 passed)
• Python compilation, VS Code diagnostics, conflict-marker scan, and CRLF-aware git diff --check passed.

Closes #1033