Add shared Gauge/GaugeNeedle fixtures for coverage guard (currently excluded)

[jeremy]

Summary

Gauge and GaugeNeedle both carry rich-text description_attachments
companion arrays (Gauge: optional non-nullable; GaugeNeedle: @required), but
neither has a shared JSON fixture under spec/fixtures/. They are exercised
only inline in go/pkg/basecamp/gauges_test.go.

Why it matters

The fixture-completeness guard (scripts/check-fixture-coverage.rb,
spec/fixtures/manifest.yaml) can only guarantee a schema keeps a validated
representative when that schema has ≥1 shared-fixture target. With no shared
fixture, Gauge / GaugeNeedle are recorded in the manifest's
excluded_schemas (reason + this issue) rather than covered_schemas — so a
future required-field change to either would not be caught by the guard.

Resolution options

1. Add shared fixtures spec/fixtures/gauges/get.json (and/or a needle-bearing
   variant) with a populated description_attachments, then move Gauge /
   GaugeNeedle from excluded_schemas to covered_schemas with pointer
   targets (root `` → Gauge; `/needles/0` → `GaugeNeedle`).
2. Or keep them excluded if the inline Go coverage is deemed sufficient and
   demand stays low — but then update this issue with that decision.

Low urgency; filed so the exclusion is tracked rather than silent.