docs: Add opencode-browser plugin to ecosystem

[benjaminshafii]

Summary Adds opencode-browser to the ecosystem page.

opencode-browser is a plugin that enables browser automation without MCP servers using your chromium-based browser.

Features:

• Works with existing Chrome/Brave/Arc profiles
• Local sockets + native messaging
• One OpenCode session can own multiple tabs and run actions in parallel
• Declarative primitives for click/type/query/snapshot

Demo: https://x.com/benjaminshafii/status/2009040618258092167?s=20

Example use case: automate admin-console tasks with existing logins and extract visible verification tokens without eval.