Document external extension reference

[0YHR0]

What changed

• linked the independent AgentMesh Extension Starter from the English and Chinese README
• documented the external distribution and entry-point discovery proof
• recorded the Daily Brief scenario, two-extension collision verification and free CI status

Why

Runtime Extension API v0.1 needed proof from a separately versioned repository rather than only an in-tree scenario. The reference repository demonstrates that extension authors can ship a wheel with a service, API, workspace, assets and health probe without changing AgentMesh core.

Validation

• external repository lint passed
• 4 extension tests passed
• extension wheel built with entry-point and assets
• temporary wheel installation discovered both agentmesh.music-studio and community.daily-brief
• external GitHub CI passed