feat(orchestrator): add hot runner protocol placeholder

Initial scaffold for the runner registration and hot editor provider module.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: frostebite

src/model/orchestrator/runners/README.md

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+# Hot Runner Protocol
+
+Extensible runner registration and persistent Unity editor provider protocol.
+
+See GitHub Issue for full specification.