Extract devcontainer export package

[codeforester]

Summary

• Move Dev Containers export ownership from base_setup into a focused base_devcontainer package.
• Keep base_setup.devcontainer_export as an explicit compatibility facade for historical imports.
• Update base_setup command dispatch to use the focused package directly.
• Add package-boundary tests and a Python package ownership doc for the staged Extract devcontainer and devenv surfaces from base_setup #1576 extraction.

Refs #1576

Validation

• /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest cli/python/base_devcontainer/tests/test_export.py cli/python/base_setup/tests/test_devcontainer_export.py cli/python/base_setup/tests/test_compatibility_facades.py -q
• /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest tests/test_base_cli_docs.py tests/test_contract_hardening.py -q
• BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash bats cli/bash/commands/basectl/tests/devcontainer.bats cli/bash/commands/basectl/tests/devenv-report.bats
• /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest cli/python/base_setup/tests/test_devenv_report.py -q
• /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest cli/python/base_devcontainer/tests/test_export.py cli/python/base_setup/tests/test_devcontainer_export.py cli/python/base_setup/tests/test_compatibility_facades.py lib/python/base_cli/tests/test_public_command_lifecycle.py -q
• /Users/rameshhp/.base.d/base/.venv/bin/python -m pylint --rcfile=.pylintrc cli/python/base_devcontainer cli/python/base_setup/devcontainer_export.py cli/python/base_setup/engine.py cli/python/base_setup/tests/test_compatibility_facades.py
• git diff --check
• env -u BASE_HOME BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash BASE_CACHE_DIR=/private/tmp/base-1576-devcontainer-full ./bin/base-test