core: commands drop selected reasoning variant

Description

V2 drops the selected reasoning variant when a command selects an agent or model without an explicit variant. For example, selecting xhigh and running a command whose agent specifies the same base model causes the reasoning indicator to disappear and the session to use the default variant.

V1 carries the selected variant name to the command-selected model when supported, falling back to that model's default otherwise. Explicit command or agent variants should still take precedence.

This appears to be an unintended consequence of the command-agent model precedence added in #34849.

Plugins

@onshape-mcp/opencode-auth

OpenCode version

0.0.0-local-202608191119 (v2)

Steps to reproduce

1. Open the V2 TUI and select reasoning effort xhigh.
2. Run a command whose configured agent uses a model without an explicit variant.
3. Observe that the reasoning indicator disappears and the session switches to the default variant.

Screenshot and/or share link

Not available.

Operating System

Linux 7.0.0-29-generic (x64)

Terminal

TERM=xterm-256color, COLORTERM=truecolor