jtk: update docs, fixtures, and golden tests for the command surface rethink

[rianjs]

Parent: #230

Part of the JTK command surface area rethink tracked in #230.

Acceptance criteria:\n- README/help text/examples match the new command surface\n- Golden tests cover stdout/stderr discipline, pagination placement, --id precedence, --fields behavior, and block-mode lists\n- Fixtures are updated to reflect the new shapes\n- Docs describe the new output model without legacy contradictions