docs: add PR template redirecting external authors to issues-only policy (#1517)

GitHub auto-populates pull_request_template.md into every new PR body,
so it steers would-be external PR authors to CONTRIBUTORS.md before they
submit a diff.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/pull_request_template.md

@@ -0,0 +1,28 @@
+<!--
+  ⚠️ Before you open this pull request, please read our contribution policy.
+
+  We accept ISSUES, NOT PULL REQUESTS from external contributors.
+
+  Design and implementation are done by the maintainers through a
+  prompt-driven workflow (see AGENTS.md). External source PRs will not be
+  merged — they have to be reverse-engineered to fit our component/theme
+  conventions and coverage gates, so it's faster for us to re-derive the
+  change from your intent.
+
+  If you've found a bug or want a feature:
+    → Open an issue on the appropriate version board instead.
+
+  If you've already built the change locally:
+    → Open an issue and share the PROMPT(S) you used to generate it, not a
+      diff. We'll reproduce it through our own workflow.
+
+  Full policy: https://github.com/modelcontextprotocol/inspector/blob/v2/main/CONTRIBUTORS.md
+
+  Maintainers: delete this template body and describe your change normally.
+-->
+
+> **Heads up:** this repository accepts **issues, not pull requests** from
+> external contributors. Please read [`CONTRIBUTORS.md`](../blob/v2/main/CONTRIBUTORS.md)
+> before continuing. If you're an external contributor, open an issue (and
+> share the prompt you used, if you've already built the change) rather than
+> this PR.