chore: add Copilot prompt for PR interface review [no-ci] (#706)

Adds `.github/prompts/pr-interface-review.md` — a Copilot prompt file
for systematic per-interface review of PRs that touch the public surface
(types, schemas, exported symbols, wire formats, CLI flags).

The prompt classifies every signature change into one of six tiers (🚨
breaking through 🆕 brand-new), flags compat measures already taken, and
calls out gaps that remain. Specifically catches subtle failure modes —
class→alias collapse, read-side vs write-side compat, default-value
flips, enum widening, async/sync flips, generic variance changes.

Canonical source and rollout context:
[`deepgram/dx-stack`](https://github.com/deepgram/dx-stack/tree/main/.github/prompts).

`[no-ci]` — docs-only addition.