chore: try and discourage slop, even a tiny bit (#18293)

i don't know how to stem the flood of slop PRs but maybe this will help
a tiny bit

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Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>

AGENTS.md

@@ -4,6 +4,8 @@ This guide is for AI coding agents working in the Svelte monorepo.
 
 **Important:** Read and follow [`CONTRIBUTING.md`](./CONTRIBUTING.md) as well - it contains essential information about testing, code structure, and contribution guidelines that applies here.
 
+When submitting a PR, you **MUST** read [`PULL_REQUEST_TEMPLATE.md`](./.github/PULL_REQUEST_TEMPLATE.md) and fill it out correctly. **DO NOT** submit a PR without running the full test suite.
+
 ## Quick Reference
 
 If asked to do a performance investigation, use the `performance-investigation` skill.