chore: clean public repo boundary

[solomonneas]

Summary

• untrack live dogfood workspace files from the public repo root while leaving them available locally through gitignore
• remove old internal planning docs, root memory cards, root hooks/scripts/skills, legacy .solo-mise fragments, and the stale solo-mise banner from the public tree
• keep public product templates under src/brigade/templates/ and tests under tests/
• track the current Brigade social preview image referenced by README.md
• fix MANIFEST.in to include src/brigade/templates instead of the old src/solo_mise/templates path

Verification

• .venv/bin/python -m pytest -q -> 262 passed
• PYTHONPATH=$HOME/repos/content-guard/src python3 -m content_guard scan . --policy $HOME/repos/content-guard/policies/public-repo.json -> Clean, 67 files checked
• git diff --cached --check -> passed
• .venv/bin/brigade doctor --target . -> passed
• .venv/bin/python -m build --sdist --wheel -> built sdist and wheel; existing setuptools license deprecation warnings only
• pre-push content guard -> Clean, 67 files checked

Note

This cleans the current public tree. It does not purge prior Git history. If any previously pushed content is truly sensitive, handle that separately with history rewrite or repository visibility controls.