feat: ask PR authors for a release-note line (AVO-1551)

Release notes derived from PR titles read like a commit log. This adds the
section an author fills in — one customer-facing line plus a type — which
becomes Avo's release notes and feeds the monthly "what shipped" rollup that
the changelog post and newsletter are written from.

No package field here, unlike the monorepo's template: every PR in this
repository is the `avo` gem, so naming it would be noise on every PR.

The check that reports on this section is not in this PR. Sharing one parser
across both repositories means reading it from avo-hq/workspace, which is
private — so a check here would need a credential that does not exist yet, and
would put private source on a public repository's runner. That tradeoff wants
a decision before it ships; the template is useful on its own in the meantime.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: Paul-Bob

.github/PULL_REQUEST_TEMPLATE.md

@@ -7,6 +7,25 @@ Fixes # (issue)
   By submitting the Contribution, you acknowledge that you have read the Contributor License Agreement at https://avohq.io/cla and agree to be bound by its terms.
 -->
 
+## Release note
+
+<!-- release-note-help
+This becomes the customer-facing line in Avo's release notes and feeds the
+monthly "what shipped" rollup. Write it for someone who uses Avo, not for a
+reviewer reading the diff.
+
+  Note  one line, no trailing period
+  Type  feature | fix | improvement | internal
+
+Use `internal` for changes users never see — CI, tooling, docs. A check
+reports a missing or unparseable note but never blocks the merge.
+
+Delete this comment once you have filled the two lines in.
+-->
+
+Note:
+Type:
+
 # Checklist:
 <!--
   Please go through the steps and complete them if they make sense (add tests if the change requires it, add to the docs, etc.)