Closes the decision-to-ticket coverage gap (#723). The `decisions-coverage`
Makefile target already reported per-decision counts; this adds the
single-decision drill-down via `tooling/db/decision show D-159`, returning
linked tickets, outbound refs, and inbound refs in one shot.
Schema unchanged — reverse link is a SELECT on tickets.decision_ref.
Refs: #723
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>