docs: establish transactional runtime identity

[wusuiling-if]

Summary

• make Transactional Agent Runtime the primary product identity
• put mca tx demo, guarantees, and limits in the first 60 README lines
• move transaction, runtime, provider, and sandbox details into focused docs

Verification

• python3 -m pytest -q tests/test_transaction.py (11 passed)
• whitespace checks passed

No CLI, package-name, or runtime behavior changes.