chore: add pull request template

[xernobyl]

What does this PR do?

Adds a default pull request template (.github/pull_request_template.md) so new PRs get a consistent description and checklist. The template is adapted from the formbricks repo and tailored to Hub:

• Conventional Commits reminder in the title
• What does this PR do? and How should this be tested? sections
• Checklist aligned with Hub: make build, make tests, make fmt/make lint, migrations, link to AGENTS.md
• Appreciated items: OpenAPI/contract tests, docs updates, coverage for logic changes

No issue linked (repo setup improvement).

How should this be tested?

• Open a new PR from any branch and confirm GitHub pre-fills the body with the new template.
• Optionally run make build, make fmt, make lint to confirm the repo is clean (no code changes in this PR).

Checklist

Required

• Filled out the "How to test" section in this PR
• Read Repository Guidelines
• Self-reviewed my own code
• Commented on my code in hard-to-understand bits (N/A — single markdown file)
• Ran make build
• Ran make tests (integration tests in tests/)
• Ran make fmt and make lint; no new warnings
• Removed debug prints / temporary logging (N/A)
• Merged the latest changes from main onto my branch with git pull origin main
• If database schema changed: added migration in migrations/ with goose annotations and ran make migrate-validate (N/A)

Appreciated

• If API changed: added or updated OpenAPI spec and ran contract tests (N/A)
• If API behavior changed: added request/response examples or Swagger UI screenshots to this PR (N/A)
• Updated docs in docs/ if changes were necessary (N/A)
• Ran make tests-coverage for meaningful logic changes (N/A)