docs: add OCX and opencode extension plugins to ecosystem

[kdcokenny]

Summary

Adds 5 community projects to the ecosystem.

Projects

• ocx – The missing package manager for OpenCode extensions – ShadCN model with Ghost Mode

Plugins

• opencode-background-agents – Claude Code-style background agents with async delegation and context persistence
• opencode-notify – Native OS notifications for OpenCode – know when tasks complete
• opencode-workspace – Bundled multi-agent orchestration harness – 16 components, one install
• opencode-worktree – Isolated branches for AI experiments – git worktrees that spawn their own terminal

All projects include the required affiliation disclaimer per README.md L97.