feat(opencode): thought level config in acp

[knotbin]

Issue for this PR

Closes #16543

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Changes thinking modes of models in ACP from being separate models with different names, e.g. OpenAI/GPT-5.3 (xhigh) and OpenAI/GPT-5.3 (medium), to just having one model listed for each, e.g. OpenAI/GPT-5.3, with a new config option ("thought") for the thought level.

How did you verify your code works?

I added a new test for config options and I tested with a local dev ACP server in the Zed code editor.

Screenshots / recordings

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

If you do not follow this template your PR will be automatically rejected.

[github-actions]

Hey! Your PR title core: thought level config in acp doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.