[FEATURE]: Add a "Set as default" action to /models to persist the current model

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before. (searched: "set default model", "persist model", "save selected model", "remember model selection"; the closest matches, #11209 and #28110, are related but different — see Additional context)

Summary

The /models selection is session-scoped by design — the models guide states "The selection applies to the current session and does not change your configuration" and "Switching a session's model does not rewrite the config file". So when the configured default is model A and I switch to model B in the TUI, there is no first-class way to make B the default for future sessions — the only option is to hand-edit opencode.json(c).

Describe the enhancement

Add a quick, discoverable way to persist the current model selection as the default, for example:

• A "Set as default" action in the /models dialog, and/or
• A command-palette command (e.g. model.set_default) that can be keybound, writing the selected provider/model to the user's global opencode.json (global rather than project-level, so personal model preferences don't get committed to shared repos)

Current behavior

1. Set "model": "provider/a" in the global opencode.json
2. Start the TUI, press /models, select provider/b
3. Work, then quit OpenCode
4. Start OpenCode again — new sessions fall back to provider/a again (occasionally a session instead resumes the last-used model, see Additional context), and there is no built-in shortcut to make provider/b the default

Expected behavior

A supported way to persist the current model as the default without manually editing the config file.

Environment

• opencode version: 2.0.7 (channel: latest)
• OS: Windows 11 Home (build 26200)
• Terminal: mintty (xterm-256color, truecolor)
• Shell: Git Bash (usr/bin/bash.exe)
• Active plugins: none configured

Additional context

• This is an explicit, opt-in "write it to config" action, not automatic remembering. #28110 (per-agent model remembering in the desktop app) and #11209 (per-agent defaults in opencode.json) both aim to change which model is picked automatically; this request is about letting the user commit their current selection as the default at a moment of their choosing.
• Observed locally: some new sessions start with the last-used model (different from the configured default), which suggests the TUI keeps internal per-agent model state (also referenced in #28110). Explicit "Set as default" semantics would make the precedence between the config default and the remembered selection predictable.

Start at the /models dialog and the command-palette entry point, then read the linked models guide to confirm the current session-only behavior and configuration boundaries. Trace how the selected provider/model and global opencode.json are represented; done means an explicit action writes the selected model globally and a new session uses it without changing project configuration.