lab(arm): wire coding_markdown_v5_reqecho_v1 (T2 requirements-echo contrast arm)

v5 chassis + MIKE_REQUIREMENTS_ECHO=1: fetch_requirements joins the
resident tool set (Glob,Grep,Read,generate_docx,fetch_requirements,Edit),
and under the exposure gate the first generate_docx refusal re-serves the
verbatim task prompt alongside the unread-coverage list (requirements echo
rides the coverage echo). echo_call_count>=1 is asserted post-run so a
completed run with a non-firing gate is typed as a failure, not silently
treated as the mechanism having run.

Delta wiring: coding_markdown_v5_reqecho_v1 joins lean_batch/coding_*/
v5-specific receipt deltas and the requirements_echo_delta include list in
all three receipt blocks (differential lever vs the 8 no-echo v5 tax
strata). tsc --noEmit clean.