docs: add pull request template with cross-repo checklist

Adds `pull_request_template.md` so every PR gets a description
prompt and a checklist reminding authors to check related surfaces:
Helm chart (separate PR), Terraform provider, and docs.

Author: mbevc1

.github/pull_request_template.md

@@ -0,0 +1,9 @@
+## Description
+
+<!-- What does this PR change and why? Link any related issue. -->
+
+## Checklist
+
+- [ ] **Helm chart** (`charts/k8s-reporter/`) checked / updated, if needed. Note: this lives in a **separate PR**
+- [ ] **Terraform provider** and related changes ([terraform-provider-kosli](https://github.com/kosli-dev/terraform-provider-kosli), [terraform-aws-evidence-reporter](https://github.com/kosli-dev/terraform-aws-evidence-reporter), [terraform-aws-kosli-reporter](https://github.com/kosli-dev/terraform-aws-kosli-reporter)) checked / updated, if needed
+- [ ] **Docs** are autogenerated from CLI help — make sure the help text reflects your changes