feat: add simplified permission system for external file access

[Mishkun]

I've vibed a solution to fix #3218 to let user to read/writes files external to cwd instead of simply denying it

Examples:

1. Ask for permission (default behavior)

{
  $schema: https://opencode.ai/config.json,
  permission: {
    edit: {
      enabled: allow,
      external_files: ask
    }
  }
}

2. Allow all external file access

{
  $schema: https://opencode.ai/config.json,
  permission: {
    edit: {
      enabled: allow,
      external_files: allow
    }
  }
}

I also asks for permission with option to allow a read for session:

Fixes #3218