Prepare base_cli runtime helpers for extraction

[codeforester]

Summary

• Move Base CLI runtime layout, directory creation, and log-retention pruning helpers into private base_cli._runtime.
• Keep base_cli.app focused on app registration, command wrapping, and context assembly.
• Add boundary tests that keep runtime helpers private and out of the public package exports.

Closes #1473

Validation

• PYTHONPATH=lib/python:cli/python $HOME/.base.d/base/.venv/bin/python -m pytest lib/python/base_cli/tests/test_app_runtime_boundary.py -q
• PYTHONPATH=lib/python:cli/python $HOME/.base.d/base/.venv/bin/python -m pytest lib/python/base_cli/tests/test_app_runtime_boundary.py lib/python/base_cli/tests/test_app_log_retention.py lib/python/base_cli/tests/test_app_runtime_errors.py -q
• PYTHONPATH=lib/python:cli/python $HOME/.base.d/base/.venv/bin/python -m pytest lib/python/base_cli/tests tests/test_base_cli_docs.py -q
• PYTHONPATH=lib/python:cli/python $HOME/.base.d/base/.venv/bin/python -m pylint lib/python/base_cli/app.py lib/python/base_cli/_runtime.py lib/python/base_cli/tests/test_app_runtime_boundary.py
• PYTHONPATH=lib/python:cli/python $HOME/.base.d/base/.venv/bin/python -m compileall -q lib/python/base_cli/app.py lib/python/base_cli/_runtime.py lib/python/base_cli/tests/test_app_runtime_boundary.py
• git diff --check
• env BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash BASE_CACHE_DIR=/private/tmp/base-1473-cli-smoke bin/basectl prompt list