docs: add pull request template

[davideast]

Adds .github/pull_request_template.md, a reviewer-facing scaffold that requires runnable consumer behavior, explicit risk, fact-based headings, and collapsed verification before a PR is posted.

REPLACE: One sentence naming what this PR changes and what it is.

```ts
// REPLACE: Runnable consumer code that defines every value it uses and shows
// the result. Change the fence language when TypeScript is not appropriate.
```

## Risk

## REPLACE: A heading that states a fact about this change

<details>
<summary>Implementation notes and verification</summary>
</details>

Risk

This changes authoring behavior only after the template lands on the default branch; existing PRs and runtime packages are untouched. The reviewer judgment is whether the ten-item completion criterion is appropriately strict for this repository, especially the requirement to demonstrate reader-visible behavior for documentation-only changes.

Every PR starts with a checkable completion criterion

1. The opening is one sentence naming the concrete change and what it is.
2. A runnable consumer example follows immediately. It defines every input,
   uses real imports and values, and explains domain terms in plain language.
3. Every changed public entry point is demonstrated through usage. Add more
   code, output, or tables when one example cannot cover the whole surface.
4. Risk appears near the top and states what could break and what needs
   reviewer judgment.
5. Every narrative heading states a fact specific to this change.
6. Code, output, or a table appears before prose that explains it.
7. Verification, findings, and declined alternatives live in <details>.
8. Coverage changes state the numeric delta and its cause, including whether
   scenarios were added, removed, or reclassified.
9. Every sentence passes the reuse test: if it could appear unchanged in a
   different PR, cut it.
10. Replace every placeholder and delete this instruction comment.

The instructions are hidden in the rendered description but remain beside the fields they govern. Authors get one completion criterion rather than separate, duplicated guidance elsewhere in the repository.

Verification remains available without becoming the review narrative

<details>
<summary>Implementation notes and verification</summary>

<!-- Commands, exit codes, test counts, findings, and coverage causes. -->

</details>

Findings and command output stay one click away while the visible description leads with consumer behavior and risk.

Implementation notes and verification

• The branch is based on 401f70f, the merge of fix(model): make Transformers peer optional #87.
• The diff adds one file at GitHub's conventional .github/pull_request_template.md path.
• git diff --check: exit 0.
• The template matches the globally guided workspace version byte-for-byte.
• Runtime tests were not run because this changes only GitHub-authored PR text.