chore(repo): Add CODEOWNERS for hotload boundary files

Author: jacekradko

.changeset/codeowners-hotload-boundary.md

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+---
+---

.github/CODEOWNERS

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+# Hotload boundary
+#
+# These files are part of the runtime surface that ships to consumer
+# apps without explicit version bumps. See AGENTS.md for context.
+
+/packages/clerk-js/src/core/clerk.ts          @clerk/team-sdk
+/packages/react/src/isomorphicClerk.ts        @clerk/team-sdk
+/packages/ui/src/contexts/                    @clerk/team-sdk