Add generic bb document import

[codeforester]

Summary

• Add parser-free bb documents import --dry-run --file ... planning.
• Add real bb documents import --file ... to record BB_DOCUMENT and canonical BB_DOCUMENT_OBJECT rows.
• Copy imported files into SHA-256 addressed financial/canonical storage with idempotent duplicate handling.
• Document the new v2 document intake command.

Validation

• uv run pytest tests/test_bb_cli.py tests/test_bb_documents_cli.py tests/test_bb_storage.py tests/test_bb_foundation.py tests/test_packaging.py -q
• ./tests/validate.sh
• git diff --check
• git diff --cached --check

Closes #132