chore: add PR template

Author: 0xgleb

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,10 @@
+## Motivation
+
+<!-- Why is this change needed? Link the issue/ADR it advances. State the problem
+     and the desired end state, not the diff. -->
+
+## Solution
+
+<!-- How does this PR solve it? Approach and key design decisions. One line per
+     bullet; detail belongs in the code or the linked issue. Note stack
+     relationships and any trade-offs or follow-ups. -->