[TypeSpec to SDK Release] Avoid duplicate release plan creation

As more services generate SDKs and manage releases through SDK Tools MCP, creating release plans has become easier—but we’re also seeing more duplicates. These duplicates confuse service teams and SDK PMs and waste SDK engineering time in reviewing and closing duplicate AutoPRs.

The expectation is SDK Tools Agent can check if any existing release plan for the same purpose - releasing SDKs for the same service or SDK package + API version, and remind user the existing release plans, instead of creating new directly.