feat: add chat memory sweep imports

[solomonneas]

Summary

• add brigade work import chat-sweep for converting nightly sweep issues into local work imports
• install a chat memory sweep JSON contract and OpenClaw memory sweep cron fragment
• document ClickClack and broader chat provider registry coverage, cron staggering around updater windows, and backup health roadmap items

Validation

• .venv/bin/python -m pytest tests/test_work_cmd.py tests/test_init.py tests/test_gitignore.py -q
• .venv/bin/python -m pytest -q
• PYTHONPATH=$HOME/repos/content-guard/src python3 -m content_guard scan . --policy $HOME/repos/content-guard/policies/public-repo.json
• git diff --check
• live brigade work import chat-sweep smoke against a temporary target