Add opencode-synced plugin to ecosystem documentation

[iHildy]

Summary

Adds opencode-synced to the ecosystem page.

opencode-synced is a plugin that enables syncing global opencode configurations across machines

Features:

• Private and Public options of config visibility (forced private in certian situations)
• Easily pull and push your config changes with commands (ex: /sync-status)

Example use case: Easily have plugin, auth, mcp, agent, etc parity between your local machine and a VPS when you use opencode on both.