navigation with arrow keys for slash commands with highlighting

[rakshith1928]

Description

Add keyboard navigation (up/down arrows) and Enter selection to the slash command completion menu, matching modern CLI UX .
Closes #575 (maybe create a new issue for mouse click feature in future)

Changes:

• Type / → completions appear with first item auto-selected (▸ /command)
• ↑/↓ → navigate completions
• Enter → selects highlighted command → fills /command in input, clears placeholders
• Tab → no-op when completions visible (This avoids conflict with file autocomplete (which uses Tab to select))->Use Enter to select
• Esc → clears input, hides completions, resets selection

Also The Completion type is defined in source/types/components.ts (line 18), not in types/index.ts. The original code had a wrong import that would fail type-checking.

Files changed:

• source/hooks/useUIState.ts - added selectedCompletionIndex state
• source/components/user-input.tsx - navigation, selection, highlighting( selectedCommad uses colors.info others use colors.primary)
• source/hooks/useUIState.spec.ts - new state tests
• source/components/user-input.spec.tsx - render tests

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Demo Video

Screen.Recording.2026-06-17.035138.mp4

Future upgarde maybe adding mouse click option .
Check issue #575

Testing

pnpm test:format
pnpm test:types
pnpm test:lint
pnpm test:ava
pnpm test:knip
pnpm test:audit
source/hooks/useUIState.spec.ts - new state tests
source/components/user-input.spec.tsx - render tests

Automated Tests

• New features include passing tests in .spec.ts/tsx files
• All existing tests pass (pnpm test:all completes successfully)
• Tests cover both success and error scenarios

Manual Testing

• Tested with Ollama
• Tested with OpenRouter
• Tested with OpenAI-compatible API
• Tested MCP integration (if applicable)

Checklist

• Code follows project style guidelines
• Self-review completed
• Documentation updated (if needed)
• No breaking changes (or clearly documented)
• Appropriate logging added using structured logging (see CONTRIBUTING.md)