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

[stevoland]

What does this PR do?

Resolves: #9296

Planned with configured GTP-5.2, handover to build errored with GPT-5.2 rather than the configured opus-4.5:

⚙ functions.glob [pattern=packages/opencode/src/**/__tests__/**/*, path=/Users/stevoland/workspace/github.com/sst/opencode]
Tool 'functions.glob' not in registry. External tools (MCP, environment) cannot be batched - call them directly. Available tools: question, bash, read, glob, grep, edit, write, task, webfetch, todowrite, todoread, skill, lsp, plan_exit, plan_enter, repo-explorer, repo-explorer_repo_ast, repo-explorer_repo_cleanup, repo-explorer_repo_clone, repo-explorer_repo_deps, repo-explorer_repo_exports, repo-explorer_repo_file, repo-explorer_repo_find, repo-explorer_repo_hotspots, repo-explorer_repo_search, repo-explorer_repo_structure, github-triage, github-pr-search
Invalid 'messages[15].tool_calls[1].function.name': string does not match pattern. Expected a string that matches the pattern '^[a-zA-Z0-9_-]+$'.
▣ Build · gpt-5.2

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

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[rekram1-node]

eh it should do:

• check if build agent has default model IF NOT then getLastModel(ctx.sessionID)

[stevoland]

@rekram1-node my bad. done and added the reverse for for build -> plan

[Antonio-Bennett]

This is needed, made a PR as well didn't see this one (closed mine)

[clement-heliot]

What is missing for this to get merged? The plan/build workflow with different configured models is broken right now

[stevoland]

@Antonio-Bennett I blew my one chance :)

You should raise yours and I'll close this, you might get lucky

[github-actions]

Closing this pull request because it has had no updates for more than 60 days. If you plan to continue working on it, feel free to reopen or open a new PR.