feat: add agent-orchestrator to community extension catalog (#2236)

Register the Intelligent Agent Orchestrator as a community extension.
Extension code is hosted externally at:
https://github.com/pragya247/spec-kit-orchestrator

Changes:
- Add agent-orchestrator entry to extensions/catalog.community.json
- Add Agent Orchestrator row to README.md community extensions table

Co-authored-by: pragya247 <pragya@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>