Handle unwritable Base cache roots without tracebacks

[codeforester]

Summary

• Convert Base runtime directory creation failures into concise Click-style errors that name the failing path.
• Include BASE_CACHE_DIR recovery guidance when cache/log/temp directories cannot be created.
• Add regression coverage for unwritable cache roots during base_cli context creation.

Issue

Fixes #895

Validation

• RED: PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest lib/python/base_cli/tests/test_base_cli.py -q -k unwritable_cache_root failed with PermissionError escaping from context creation.
• PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest lib/python/base_cli/tests/test_app_runtime_errors.py lib/python/base_cli/tests/test_base_cli.py -q
• PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest lib/python/base_cli/tests -q
• /usr/local/bin/bash -lc "git ls-files -z '*.py' | PYTHONPATH=lib/python:cli/python xargs -0 /Users/rameshhp/.base.d/base/.venv/bin/python -m pylint --rcfile=.pylintrc"\n- BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash BASE_CACHE_DIR=/private/tmp/base-unwritable-cache-895.hSKuta bin/basectl workspace status --workspace /tmp --format json exits 1 with a concise error and no traceback.\n- git diff --check origin/master\n- env -u BASE_HOME BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash ./bin/base-test\n\n## Demo Impact\nNone. Runtime error handling only.\n\n## Notes\n- .ai-context/ unchanged; this aligns an internal runtime failure path with existing error-reporting expectations without changing the command surface.