Improve PR template with detailed sections and guidance

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,11 +1,33 @@
+<!-- Heads up: this repo has two live base branches. Check that the base above is
+     the one you meant (`master` vs `tailwind-redesign`) before opening. -->
+
 Fixes #
+<!-- A GitHub issue number, or a Sentry short ID like NOTEBOOK-AI-TX.
+     Delete this line entirely if the PR isn't tied to one. -->
+
+## User-facing changes
+
+<!-- One bullet per thing a user would notice, written the way it'd read in the
+     release notes: plain language, no file or class names. These get lifted
+     straight into the GitHub release, which is our changelog.
+     Write "None (internal only)" for refactors, dependency bumps, and infra work. -->
+
+ -
 
-Changes proposed:
+## Implementation notes
+
+<!-- What you changed and why, for whoever reviews it: approach, trade-offs,
+     anything you deliberately left out or want a second opinion on. -->
 
- - 
-
  -
-
+
+## Testing
+
+<!-- Tests you added, plus anything you verified by hand. If you hit failures that
+     already exist on the base branch, say so here so nobody chases them. -->
+
  -
 
-@indentlabs/contributors
+## Screenshots
+
+<!-- Before/after for anything visual. Delete this section if there's nothing to show. -->