Extract workspace clone command

[codeforester]

Fixes #1462

Summary

• move workspace clone orchestration from base_projects.engine into base_projects.workspace_clone_command
• share workspace root and manifest-option resolution through base_projects.workspace_context
• keep engine as the compatibility facade for existing clone helper imports

Validation

• /private/tmp/base-python-train-venv/bin/python -m pytest cli/python/base_projects/tests/test_engine_structure.py -q
• /private/tmp/base-python-train-venv/bin/python -m pytest cli/python/base_projects/tests/test_workspace_clone.py cli/python/base_projects/tests/test_workspace_init.py cli/python/base_projects/tests/test_workspace_configure.py cli/python/base_projects/tests/test_workspace_pull.py -q
• /private/tmp/base-python-train-venv/bin/python -m pytest cli/python/base_projects/tests -q
• /private/tmp/base-python-train-venv/bin/pylint cli/python/base_projects/engine.py cli/python/base_projects/workspace_clone_command.py cli/python/base_projects/workspace_context.py cli/python/base_projects/tests/test_engine_structure.py
• /private/tmp/base-python-train-venv/bin/python -m compileall cli/python/base_projects
• git diff --check
• env BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash BASE_CACHE_DIR=/private/tmp/base-1462-workspace-smoke bin/basectl workspace clone --workspace /Users/rameshhp/work --manifest /Users/rameshhp/work/base-workspace/workspace.yaml --dry-run