v0.6.6 — health lane hardening (single selection fn, leftover-only reaping, reason tokens)

Health lane (#76): one candidate_source() for preflight and selection; stop_remaining_tree() signals only live leftovers after a normal exit; timebox boundary re-check; Makefile detection ignores test:= and refuses to run without make (no fabricated finding); IFS/nullglob restored; error() takes a short reason token (health.json.reason machine-readable); finish() still writes health.json when the evidence manifest is unwritable.

Evidence: PR #30 (completion record; Kimi/Grok/Opus GO), guard run https://github.com/caty-ai/alpha-nightshift/actions/runs/34011132038