docs: revise deployment strategy to per-cycle branches with scripted tagging

Replace long-lived deployed/testnet and deployed/mainnet branches with
short-lived deployment/YYYY-MM-DD branches branched fresh from main per
cycle. Each cycle deploys testnet then mainnet from the same branch and
merges back to main when it closes; tags (deploy/<env>/YYYY-MM-DD/<name>)
are the permanent record.

- Add packages/deployment/scripts/tag-deployment.sh: scripted tag creation
  with annotated body (deployer, network, commit, per-address-book
  changes). Signed by default, preview and confirmation prompted, --base
  defaults to latest deploy/<env>/* tag, --yes for non-interactive.
- Drop .github/workflows/deployment-tag.yml and backport-artifacts.yml;
  subsumed by scripted tagging and the cycle merge-back PR respectively.
- Restate audit integrity as a byte-equality property; sketch an audit-
  integrity CI check as future work in Appendix A, with prerequisite to
  reorganize non-production Solidity into clearly-scoped directories.

Author: RembrandtK