feat: add model/agent slot keybinds for direct switching

[adam2am]

Summary

• Add model_slot_1-5 keybinds to jump directly to favorite models
• Add agent_slot_1-5 keybinds to jump directly to agents by index
• Add setFavorite(index) method to model store
• Add setSlot(index) method to agent store

Motivation

Currently switching models requires cycling through all favorites with Tab/F2. This PR enables direct keybind mapping for faster workflow - press F1 for favorite model 1, f5 for agent 1, etc.

Example Config

"keybinds": {
  "model_cycle_recent": "none",
  "model_slot_1": "f1",
  "model_slot_2": "f2", 
  "model_slot_3": "f3",
  
  "agent_slot_1": "f5",
  "agent_slot_2": "f6"
}

Files Changed

• packages/opencode/src/config/config.ts - Add 10 keybind entries
• packages/opencode/src/cli/cmd/tui/context/local.tsx - Add setFavorite/setSlot methods
• packages/opencode/src/cli/cmd/tui/app.tsx - Register slot commands