Add PR template with description, testing, and AI disclosure sections

[mnriem]

Problem

No .github/PULL_REQUEST_TEMPLATE.md exists. CONTRIBUTING.md has excellent especially the AI disclosure but none of it surfaces when someone opens a PR. Contributors have to independently discover and read CONTRIBUTING.md.policy guidance

Proposed Solution

Create .github/PULL_REQUEST_TEMPLATE.md with sections for:

• ** what the PR does and whyDescription**
• ** checklist aligned with the project's tooling (uv run specify --help, uv run pytest, sample project testing)Testing**
• AI checkboxes for disclosing AI assistance, with a link back to the AI contributions policy in CONTRIBUTING.mdDisclosure

Why

This ensures every PR author is prompted to describe their changes, confirm testing, and disclose AI without needing to separately find and read CONTRIBUTING.md.usage

[mnriem]

Closed by PR #1634