feat: Make recent models display and storage limit limit configurable (resolves #5533)

[ariane-emory]

This resolves #5533 by making the number of recent models that are stored and displayed consistent, with a default of 5.

Additionally, an optional tui.recent_models_count setting that can be set in opencode.json is added that a user may choose to use if they'd prefer a different limit.

• Add recent_models_count field to TUI config schema with default of 5
• Replace hardcoded limits (10 for storage, 5 for display) with configurable value
• Fixes confusing UX where F2/Shift+F2 cycled through models not visible in menu
• Users can now configure via tui.recent_models_count in opencode.json

Resolves #5533