[FEATURE]:Feat: Add an extension/plugin system (like VS Code)

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Describe the enhancement you want to request

Feature Request: Extension Marketplace / Plugin System (like VS Code)

Is your feature request related to a problem? Please describe.

Currently, OpenCode is a monolithic tool. Every new feature or integration must be built directly into the core repository. This limits community contributions to core modifications and increases the maintenance burden for the core team.

Describe the solution you'd like

I would like OpenCode to support a modular extension/plugin system, similar to VS Code.
This system should allow users to:

1. Download, install, and update extensions from a central registry or directly from GitHub URLs.
2. Develop third-party plugins (e.g., custom AI models, themes, specific language linters, or workspace tools) without modifying the OpenCode core.
3. Enable/disable extensions dynamically via a configuration file or a dedicated CLI command (e.g., opencode ext install <extension-name>).

Describe alternatives you've considered

An alternative would be to continue adding everything to the core, but this will eventually make the project too heavy and harder to maintain.

Additional context

Implementing a plugin architecture (using a hook/event system or a standard runtime like Node/Python plugins) would dramatically accelerate community adoption and ecosystem growth.