bug: docs do not align with behavior of instructions #1240

The instructions field is an array of file names to read instructions from so doing "instructions": ["{file:./custom-instructions.md}"] won't work. The instructions were built before support of file: prefixes so maybe they should change

Not sure if the docs should be fixed or the behavior should be updated

### Files

Use `{file:path/to/file}` to substitute the contents of a file:

```json title="opencode.json"
{
  "$schema": "https://opencode.ai/config.json",
  "instructions": ["{file:./custom-instructions.md}"],
  "provider": {
    "openai": {
      "options": {
        "apiKey": "{file:~/.secrets/openai-key}"
      }
    }
  }
}

File paths can be:

• Relative to the config file directory
• Or absolute paths starting with / or ~

These are useful for:

• Keeping sensitive data like API keys in separate files.
• Including large instruction files without cluttering your config.
• Sharing common configuration snippets across multiple config files.