chore: add PR template

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,18 @@
+## What
+
+<!-- One or two paragraphs (or bullets) that orient a reviewer to the change.
+     Specific filenames/symbols when load-bearing. -->
+
+## Why
+
+<!-- The rationale: what problem this solves, what trade-offs you made, what
+     constraints shaped the call. -->
+
+<!--
+Optional: add a `## Test plan` section below for behavioural changes not
+covered by CI — UI tweaks verified on a preview, manual flows, OAuth
+round-trips. Omit for diff-obvious changes CI already covers.
+
+Do not add other sections (Risks, Follow-ups, Screenshots, etc.) — keep
+the PR body shape uniform across LLMATIONS repos.
+-->