OPENCODE_API_KEY fallback feature

[weizhoublue]

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Type of change

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

What does this PR do?

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• I have not included unrelated changes in this PR

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[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

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[github-actions]

Hey! Your PR title fallback feature 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.