feat: add /tui and /config slash commands

[Oerum]

Issue for this PR

Closes #16911

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This PR introduces two new slash commands to the OpenCode TUI:

• /tui (aliases: tuisettings) -> opens tui.json
• /config (aliases: settings) -> opens opencode.json

Implementation details:

1. It uses the Global.Path.config utility to resolve the correct, OS-specific path for both configuration files.
2. It uses Node's fs.existsSync() to verify if the file exists. If it does not exist, it creates a blank file referencing the correct $schema so that code intelligence works immediately in the user's editor.
3. It uses the open package to launch the system's default JSON editor cross-platform.

How did you verify your code works?

• Manually verified the slash commands populate in the TUI autocomplete menu.
• Tested execution; the tui.json file correctly opens in the system's default editor.
• Tested fallback behavior where the file does not exist (the schema file is properly created and opened).
• Ran bun turbo typecheck locally to ensure no typing regressions.
• Ran bun run test across the suite to verify no breakages.

Screenshots / recordings

N/A

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR