feat: Add support for json schema in run command: opencode run --json <json schema>

[richardanaya]

Fixes #9320

What does this PR do?

Allows you to constrain run commands output to a specific JSON schema.

• Inspired by llama.cpp's functionality
• Utilizes passed in model
• Throws error if used with "--format json"

How did you verify your code works?

[github-actions]

Hey! Your PR title Add support for json schema in opencode run 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]

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

Potential Duplicate Found:

feat: support claude agent SDK-style structured outputs in the OpenCode SDK (#8161)

This PR appears related as it deals with structured outputs/schema support, which is similar in scope to the JSON schema constraint feature in PR #9319. Both are about constraining model output to specific formats/schemas.

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Possibly Related (Different Scope):

feat: add JSON Schema as source of truth for type definitions (#6987)

This PR also mentions JSON Schema but appears to be focused on using JSON Schema for type definitions rather than runtime output constraints.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.