feat(command): add /exit that exits the tui

[remarkablemark]

What is the motivation for this pull request?

Feature: adds a /exit slash command to exit the application from within the TUI.

What is the current behavior?

Users can only exit via Ctrl+C.

What is the new behavior?

Typing /exit exits the application via Ink's useApp().exit().

Checklist:

• Conventional Commits
• Tests
• Documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/components/App.tsx	100.00% <100.00%> (ø)
src/constants/command.ts	100.00% <ø> (ø)

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