Fix canonical dxt expansion

[sabino]

Summary

• change the canonical expansion to Data eXecution & Transformation
• update AGENTS.md, PLAN.md, README.md, and root CLI help text
• add a root-help regression test for the canonical name

Validation

• zig fmt --check src/root.zig
• zig build --cache-dir /tmp/dxt-zig-cache-local --global-cache-dir /tmp/dxt-zig-cache-global
• pytest -q tests/test_cli.py::test_root_help_uses_canonical_name
• python scripts/check_public_safety.py
• python scripts/check_runtime_boundary.py
• git diff --check