Add a0-llmfallback plugin

[Nunezchef]

Summary

• adds new index entry: plugins/a0-llmfallback/plugin.yaml
• points to standalone plugin repo: https://github.com/Nunezchef/a0-llmfallback

Validation

• follows index schema (title, description, github, tags)
• one plugin folder added in this PR

[frdel]

Hello, I need to warn you that this approach to plugins will soon break. Plugins are self-contained, there is no need for manual install and uninstall scripts and cloning git repos at runtime will be flagged as dangerous by our scanner. Extensions, tools, prompts etc. should be contained in the plugin directly.