feat(opencode): add runtime model override for task tool subagents

[pratikranaa]

Issue for this PR

Closes #17595, #32730

Type of change

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

What does this PR do?

Adds an optional model parameter to the task tool so a primary agent can pick a subagent model at runtime using provider/model-id format.

Model priority: explicit model param → subagent configured model frontmatter → parent assistant message model.

Explicit overrides are gated behind a new model_override permission (defaults to ask) to prevent prompt-injection escalation to expensive models.

How did you verify your code works?

• 25 tests in test/tool/task.test.ts (7 new) — all pass
• bunx tsc --noEmit clean on changed files
• Tests cover: per-agent model resolution, cross-provider models, parent fallback, variant clearing, runtime override, override priority over configured model, invalid format rejection, permission prompting

Screenshots / recordings

N/A — backend logic change, no UI.

Checklist

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

Note: #29447 implements the same feature but has been waiting for review since May. This is an independent implementation.

[github-actions]

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

Potential Duplicate Found

PR #29447: feat(opencode): add task model override
#29447

Why it's related: This is the exact same feature mentioned in the PR description. PR #29447 has been waiting for review since May and implements the same task tool model override functionality. The current PR (35800) is an independent implementation of the same feature.

[github-actions]

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

[TTTPOB]

really need this, i don't have to write a bunch of special agent def files if this get merged.

[colinmollenhour]

@pratikranaa It's not clear to me skimming the code if the variant can also be overridden? I think definitely all of these should be possible:

• Call subagent with different model - default variant
• Call subagent with different model - specific variant
• Call subagent with same model - specific other variant

For example, the parent model could be "xhigh" and call a "low" subagent or vice versa. If this is possible already, I think the docs/tool should be updated so it's more visible.