refactor(opencode): suppress hidden subagent updates

[flowluap]

Issue for this PR

Closes#37896

Type of change

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

What does this PR do?

Suppresses stream.subagent footer replacements when an event changes only the accumulated detail of an unselected child. The reducer still retains complete history, and selection immediately emits that history. Tabs, completion, cancellation, errors, permissions, questions, blockers, and selected-child detail remain immediate.

A representative 250-delta sequence reduced unselected detail-only snapshots from 251 to 0. This is an event-count result, not an end-to-end timing percentage.

PR #36046 also touches the transport for permission routing but does not implement this suppression, so it may require a small rebase.

How did you verify your code works?

• focused subagent/transport tests: 38 pass
• transport suite repeated 10 times: 330 pass
• package typecheck
• Prettier and git diff --check
• independent concurrency and state-transition review

Screenshots / recordings

Not applicable; visible footer state is unchanged.

Checklist

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

[github-actions]

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

No duplicate PRs found.

Note: PR #36046 (show permission prompts from nested subagent chains) appears in results and is mentioned in the PR description as potentially requiring a small rebase, but it's not a duplicate—it's a related PR that touches the transport for permission routing.

[github-actions]

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