Prepare bbackup 1.8.0 release

[CruxExperts]

Summary

• harden backup integrity, restore preflight, encryption fail-closed behavior, and temp-to-final storage promotion
• bump release metadata to 1.8.0 and refresh generated CLI skill documentation
• modernize GitHub-facing docs with a generated hero image, Mermaid backup/release diagrams, and documented visual/charting standards

Validation

• uv sync --locked
• uv run python scripts/check_version_sync.py
• uv run python scripts/check_publishing_ready.py
• uv run python scripts/generate_cli_skills.py --check
• uv run python -m py_compile bbackup.py bbman.py bbackup/*.py bbackup/data/*.py bbackup/management/*.py scripts/*.py
• uv build
• uv run python scripts/smoke_installed_artifact.py
• uv run pytest (513 passed)
• git diff --check
• repo secret/path scan reviewed; matches are expected examples, tests, docs, and workflow placeholders

Release Notes

Prepared as release candidate v1.8.0. Cut the final tag after review and CI are green.