[Enhancement] Add configurable keybinds for dialog actions (#6144)

[CasualDeveloper]

Problem

Several dialog keybinds are hardcoded and cannot be customized:

• Session list: ctrl+d (delete), ctrl+r (rename)
• Stash dialog: ctrl+d (delete)
• Model dialog: ctrl+a (provider list), ctrl+f (favorite toggle)

Users who have conflicts with these keybinds (e.g., ctrl+d for shell EOF, ctrl+r for reverse search) cannot remap them.

Proposed Solution

Add new keybind config options:

• session_delete - default ctrl+d
• session_rename - default ctrl+r
• stash_delete - default ctrl+d
• model_provider_list - default ctrl+a
• model_favorite_toggle - default ctrl+f

Usage

{
  "keybinds": {
    "session_delete": "super+d",
    "session_rename": "super+r"
  }
}

Files to Modify

• packages/opencode/src/config/config.ts
• packages/opencode/src/cli/cmd/tui/component/dialog-session-list.tsx
• packages/opencode/src/cli/cmd/tui/component/dialog-stash.tsx
• packages/opencode/src/cli/cmd/tui/component/dialog-model.tsx
• packages/opencode/src/util/keybind.ts