fix: experimental plan mode build incorrectly uses plan's model

[stevoland]

What does this PR do?

Resolves: https://github.com/anomalyco/opencode/issues/9296

How did you verify your code works?

 ./packages/opencode/script/build.ts --single
 ./packages/opencode/dist/opencode-darwin-arm64/bin/opencode

repeated previous prompt and build agent model was correctly invoked