[FEATURE]: add "@" mention files in prompt support in opencode web ui prompt window #8223

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Description

Add support for "@" mention syntax to reference and include files directly in the prompt window of the OpenCode web UI. This would allow users to easily reference specific files when writing prompts without manually copying file paths or content.

Use Case

When crafting prompts for code generation, debugging, or analysis, users often need to reference specific files from their project. Currently, they must manually type file paths or copy-paste content. An "@" mention system would streamline this workflow by:

• Providing autocomplete suggestions for files in the project
• Allowing users to reference files with a simple "@filename" syntax
• Automatically including file context in the prompt

Expected Behavior

1. User types "@" in the prompt window
2. A dropdown appears showing available files in the project
3. User selects a file from the list
4. The file reference is inserted into the prompt (e.g., "@src/main.js")
5. When the prompt is submitted, the referenced file's content or path is included in the context

Benefits

• Faster workflow for developers
• Reduced manual effort in referencing files
• Better context awareness for AI-assisted coding
• Improved user experience similar to mention systems in other tools

Additional Context

This feature would be particularly useful for:

• Code review prompts
• Bug fixing requests
• Feature implementation guidance
• Documentation generation