chore: add pull request template

Adds .github/pull_request_template.md with sections for summary, type
of change, testing checklist tied to the repo's pnpm scripts, a
schema-changes block for src/slack-block-kit.schema.json updates, and
a Conventional Commits reminder so release-please continues to pick up
versions correctly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: zhawtof

.github/pull_request_template.md

@@ -0,0 +1,39 @@
+## Summary
+
+<!-- What does this PR change and why? Link any related issues. -->
+
+## Type of change
+
+- [ ] Bug fix (non-breaking change that fixes an issue)
+- [ ] New feature (non-breaking change that adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing behavior to change)
+- [ ] Schema update (changes to `src/slack-block-kit.schema.json`)
+- [ ] Docs / chore (no runtime change)
+
+## Changes
+
+<!-- Bullet list of notable changes. Call out anything reviewers should pay extra attention to. -->
+
+-
+
+## Testing
+
+- [ ] `pnpm test` passes
+- [ ] `pnpm typecheck` passes
+- [ ] `pnpm lint` passes
+- [ ] `pnpm validate-schema` passes (if the schema changed)
+- [ ] Added or updated tests under `test/` covering the change
+
+## Schema changes
+
+<!-- Fill out if `src/slack-block-kit.schema.json` was modified. -->
+
+- [ ] Verified against the [Slack Block Kit reference](https://api.slack.com/reference/block-kit)
+- [ ] Backwards compatible (existing valid payloads still validate)
+- [ ] If breaking, called out in the summary and a release note is planned
+
+## Checklist
+
+- [ ] Commit messages follow [Conventional Commits](https://www.conventionalcommits.org/) (required for release-please)
+- [ ] Public API changes are reflected in `README.md`
+- [ ] No secrets, tokens, or sample tenant data committed