chore: refine Copilot PR interface review prompt [no-ci] (#707)

Refines `.github/prompts/pr-interface-review.md` based on Copilot review
feedback raised during the [original
rollout](https://github.com/deepgram/dx-stack/pull/3):

1. **Step 1 / Step 5 consistency** — Step 1 said *"ignore tests except
deleted"* but Step 5 also requires reviewing **added** tests that prove
compat shims. Broadened Step 1 to cover added, deleted, and
modified-to-remove tests. (raised on
[`deepgram/deepgram-go-sdk#329`](https://github.com/deepgram/deepgram-go-sdk/pull/329))

2. **Tier 6 broadened** — original wording only mentioned same-named
local symbols as collision vectors. Now also covers glob/namespace
imports (Python `from X import *`, Rust `use X::*`, Go dot-imports,
TypeScript `import * as` destructuring). (raised on
[`deepgram/deepgram-js-sdk#495`](https://github.com/deepgram/deepgram-js-sdk/pull/495))

3. **Fence language** — the template fenced block now declares
`markdown` to satisfy markdownlint MD040. (raised on
[`deepgram/deepgram-go-sdk#329`](https://github.com/deepgram/deepgram-go-sdk/pull/329))

Canonical source:
[`deepgram/dx-stack`](https://github.com/deepgram/dx-stack/blob/main/.github/prompts/pr-interface-review.md).
Rollout context:
[`docs/copilot-prompts.md`](https://github.com/deepgram/dx-stack/blob/main/docs/copilot-prompts.md).

`[no-ci]` — docs-only refinement to a previously-merged file.