docs(ecosystem): add opencode-zellij-namer plugin

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Summary

Adds opencode-zellij-namer to the ecosystem plugins list.

This OpenCode plugin automatically renames Zellij terminal sessions based on your work context (project name, intent like feat/fix/debug, and optional tag).

Features:

• AI-powered naming using Gemini Flash (with heuristic fallback)
• Reads AGENTS.md for project-specific naming guidance
• Non-blocking with debounce and cooldown
• Published on npm: opencode-zellij-namer