Gate external PRs on an assigned, linked issue (#3291)

Signed-off-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

.github/pull_request_template.md

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+<!--
+Pull requests from outside the maintainer team need to link an open issue that
+a maintainer has assigned to you (or one labeled `help wanted`); others are
+closed automatically until that's in place. See CONTRIBUTING.md for details.
+-->
+
+Fixes #
+
+<!-- Provide a brief summary of your changes -->
+
+## Motivation and Context
+<!-- Why is this change needed? What problem does it solve? -->
+
+## How Has This Been Tested?
+<!-- Have you tested this in a real application? Which scenarios were tested? -->
+
+## Breaking Changes
+<!-- Will users need to update their code or configurations? -->
+
+## Types of changes
+<!-- What types of changes does your code introduce? Put an `x` in all the boxes that apply: -->
+- [ ] Bug fix (non-breaking change which fixes an issue)
+- [ ] New feature (non-breaking change which adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing functionality to change)
+- [ ] Documentation update
+
+## Checklist
+<!-- Go over all the following points, and put an `x` in all the boxes that apply. -->
+- [ ] I am assigned to the linked issue (or it is labeled `help wanted`, or I'm a maintainer)
+- [ ] I have disclosed any AI assistance and can explain the change in my own words
+- [ ] I have read the [MCP Documentation](https://modelcontextprotocol.io)
+- [ ] My code follows the repository's style guidelines
+- [ ] New and existing tests pass locally
+- [ ] I have added appropriate error handling
+- [ ] I have added or updated documentation as needed
+
+## Additional context
+<!-- Add any other context, implementation notes, or design decisions -->