opentui: port flags (-m, -a)

[veracioux]

Prolly gonna rebase and resolve conflicts that are likely to arise with the opentui branch.

• Side effect: Added validation of model, which prevents a crash when you select an agent with a nonexistent model

(still need to test thoroughly)

Test matrix:

• bun dev
• bun dev --agent existing
• bun dev --agent nonexistent # warning toast
• bun dev --agent existing-with-wrong-model # warning toast
• bun dev --model existing
• bun dev --model existing --agent existing
• bun dev --model existing --agent nonexistent # warning toast
• bun dev --model existing --agent existing-with-wrong-model # warning toast
• bun dev --model nonexistent # warning toast
• bun dev --model nonexistent --agent existing # warning toast
• bun dev --model nonexistent --agent nonexistent # warning toast
• bun dev --model nonexistent --agent existing-with-wrong-model # warning toast