ci: pull request template

Prompts contributors to describe the change, enumerate per-area
impacts, document testing (including any workflow updates), and note
documentation changes. Placeholders are visible italic text so any
section a contributor forgets to replace stands out in the rendered PR.

Author: alandefreitas

.github/pull_request_template.md

@@ -0,0 +1,25 @@
+<!-- Fill this in yourself, or at least have your favorite AI agent draft it from the diff. -->
+
+_What this PR does and why. For large or non-trivial changes, link a design doc or issue, or explain the design inline — the larger the change, the more context belongs here._
+
+## Changes
+
+_Replace the lines that apply, delete the rest._
+
+- **Source**: _Implementation changes._
+- **Tests**: _New or updated unit tests and fixtures._
+- **Golden tests**: _Intentional output changes or regenerations._
+- **Build**: _Build-system, packaging, or schema changes._
+- **CI**: _Workflows or automation._
+- **Toolchain**: _Bootstrap or environment tooling._
+- **Tooling**: _Developer tools and helper scripts._
+- **Third-party**: _Vendored dependencies or patches._
+- **Breaking changes**: _Anything downstream users need to know._
+
+## Testing
+
+_How this change stays tested going forward — the tests in this PR that cover the new behavior, plus any CI workflow changes needed so that coverage runs on every future build._
+
+## Documentation
+
+_What was updated in the documentation or why documentation is not needed._