feat: add issue-backed work task flow

[solomonneas]

Summary

• Add local work task templates with default acceptance criteria and task planning guidance.
• Add optional GitHub issue import through the existing gh issue view command.
• Surface issue-backed task context in work brief and add work doctor warnings for missing acceptance, unchecked or closed issues, and stale active sessions.

Verification

• PYTHONPATH=src python3 -m pytest tests/test_work_cmd.py -q
• PYTHONPATH=src python3 -m pytest -q
• git diff --check
• PYTHONPATH=$HOME/repos/content-guard/src python3 -m content_guard scan . --policy $HOME/repos/content-guard/policies/public-repo.json
• temp repo CLI smoke for brigade work task add --template docs --acceptance ... and brigade work task plan