This PR introduces the following type:
```typescript
export type FileOpenerScheme = "vscode" | "cursor" | "windsurf";
```
and uses it as the new type for a `fileOpener` option in `config.json`.
If set, this will be used to linkify file annotations in the output
using the URI-based file opener supported in VS Code-based IDEs.
Currently, this does not pass:
Updated `codex-cli/tests/markdown.test.tsx` to verify the new behavior.
Note it required mocking `supports-hyperlinks` and temporarily modifying
`chalk.level` to yield the desired output.
