feat: add work task ledger

[solomonneas]

Summary

• add a gitignored local work task ledger under .brigade/work/tasks.json
• add brigade work tasks plus brigade work task add/show/done
• make work brief, work next, and work run prefer pending ledger tasks before dogfood fallbacks
• mark consumed ledger tasks done after successful work run executions

Verification

• .venv/bin/python -m pytest tests/test_work_cmd.py -q
• live smoke: work task add, work tasks, work brief, work task show, work task done
• .venv/bin/python -m pytest -q
• PYTHONPATH=/home/clawdbot/repos/content-guard/src python3 -m content_guard scan . --policy /home/clawdbot/repos/content-guard/policies/public-repo.json
• git diff --check