docs: add manual test case documentation for all MDL command areas

19 test case documents covering entities, enumerations, microflows,
nanoflows, pages, integration, security, navigation, organization,
workflows, catalog, tooling, CLI commands, SQL, session management,
mappings, business events, image collections, and agent/editor.

Includes AGENT-TESTING.md with shared execution methodology:
build, fixture copies, REPL testing via tmux (/ terminator convention),
Docker-dependent tests, auth tests, verification patterns, and
result reporting format.

Author: retran