Let base_cli testing write manifest fixtures

[codeforester]

Summary

• Add keyword-only manifest={...} support to base_cli.testing.invoke.
• Write base_manifest.yaml into cwd before invoking project-aware commands.
• Document the fixture pattern in the base_cli README.

Validation

• PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest lib/python/base_cli/tests/test_testing.py -q
• PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest lib/python/base_cli/tests -q
• PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pylint --rcfile=.pylintrc lib/python/base_cli/testing.py lib/python/base_cli/tests/test_testing.py
• git diff --check

Project Fields

• Priority: P2
• Size: S
• Area: Python
• Initiative: Adoption Polish

Fixes #942