tui (v2): config default model ignored, falls back to first provider's model

Description

Bug in OpenCode v2 (opencode2), the beta build. The TUI ignores the
config-level default model and falls back to the first provider's model when
initializing the build agent. The displayed model is then sent with
session.create / session.prompt, so new sessions actually run on the wrong
model.

Repro: config sets "model": "custom-provider/my-default-model", but the TUI
shows and uses a model from a different provider that happens to be first in
the provider list (e.g. github-copilot/gemini-3.7-flash).

Root cause (client-side): sync.data.config is the raw array returned by
/api/config (an array of config sources), so sync.data.config.model is
always undefined and the if (sync.data.config.model) check in
packages/tui/src/context/local.tsx never matches. The fallback then picks
provider[0]'s default model.

TUI vs API code gap: the server/API path resolves the configured default
correctly — packages/core/src/session/runner/model.ts uses
catalog.model.default() when a session has no model, and GET /api/model/default
returns the configured model. The TUI client has no equivalent: it never
flattens the /api/config source array into a config object, so the
config-level model is invisible to the client-side default-model selection.
The server-side /api/model/default and the TUI's sync.data.config.model
therefore disagree.

Plugins

None

OpenCode version

opencode2 v0.0.0-beta-17519 (v2 beta)

Steps to reproduce

1. Set "model": "custom-provider/my-default-model" in ~/.config/opencode/opencode.jsonc (any custom provider + model).
2. Have the github-copilot provider registered (gh CLI authenticated, ~/.config/gh present) so it appears early in the provider list.
3. Run opencode2 (TUI).
4. Build agent's initial model shows the github-copilot model instead of the configured custom-provider/my-default-model.
5. Send a prompt — the session runs on the github-copilot model (passed via session.prompt).

Screenshot and/or share link

N/A

Operating System

Ubuntu 24.04 (Linux 5.15.0-91-generic x86_64)

Terminal

tmux (TERM=xterm-256color)