feat(opencode): add GMI Cloud request identifier

[isaachuangGMICLOUD]

Issue for this PR

Related to anomalyco/models.dev#1724

Type of change

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

What does this PR do?

Adds a GMI Cloud-specific User-Agent header so GMI Cloud can identify requests coming from opencode.

The header is applied only to the gmicloud provider through the existing provider custom options path.

Header format:

User-Agent: opencode/<version> gmicloud (<platform> <release>; <arch>)

How did you verify your code works?

• Ran a local request with gmicloud/deepseek-ai/DeepSeek-V4-Flash
• Confirmed the request returned gmi-opencode-header-test
• Confirmed GMI Cloud could track the request by header
• bun turbo typecheck passed during pre-push

Screenshots / recordings

Not a UI change.

Checklist

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

[github-actions]

Hey! Your PR title Add GMI Cloud request identifier 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.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍