fix(lab): echo joins consolidated v5 (never a separate arm); triage prompt tightened (gen-4)

Correction from Eli: the exposure echo was always meant to live IN the
consolidated arm - testing it as a standalone contrast arm was exactly
the trivial single-lever A/B the no-multiplicative-arms doctrine bans.
coding_markdown_v5_echo is deleted; coding_markdown_v5 now carries
MIKE_EXPOSURE_ECHO=1 (ablation happens via the exposure_echo receipt
flag over run strata, not via forked arms).

Prompt gen-4: the CORPUS WORKFLOW block compressed 5 bullets -> 3
(~40% shorter) keeping every measured directive (survey-first corpus
grep, fair split, scoped reads, bare filenames, batching, coverage
floor, full-central-past-truncation, corpus-gap noting, verify-at-
source, interactions-together, anti-skip damping). Dropped only the
scarcity exposition - foregrounding the budget fed the documented
context-anxiety skip response the block exists to prevent.

Preflight: gen-4 prompt sha 645ef951, tool sha c8859b8e unchanged;
tsc clean. In-flight gen-3 flash rows remain valid no-echo/long-prompt
strata, disambiguated by prompt sha + exposure_echo flag.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pfUVhNFTRvhYGXBwoKNj6