Completes ADR-0174 §6's observability rollout and adds the outbound trust
boundary that ADR-1577's inbound boundary has no counterpart for.
D1 (wire the seam behind the existing opt-in gate) shipped via #2620 / PR
#2621. D1b records that the unconditional stderr-on-error rule at 0174:105
is the target state, deferred behind an explicit --json-errors envelope
version plus migration note -- disclosed as a partial supersede rather than
retconned. D2-D5 are Directional; D6's non-goals are binding.
Regenerates docs/adr/README.md via scripts/gen-adr-index.cjs --write.
Co-authored-by: Tom Boucher <trekkie@nomorestars.com>
