fix(cli): clarify background Task orchestration

[marius-kilocode]

Background Task execution already returns immediately and automatically retriggers the parent when a result arrives. The issue was ambiguous guidance that did not clearly distinguish work that can run independently from work whose result is needed before the parent proceeds.

This change makes the Task guidance generic:

• Use foreground when the next step depends on the result.
• Use background only while useful non-overlapping work remains.
• Do not give the final answer until all required background results have arrived.

The change intentionally does not alter the Task lifecycle, notification payloads, runtime finish behavior, or add a tracking registry. The investigation and public-harness comparison are documented in #13360.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (2 files)

• .changeset/clear-background-task-waiting.md
• packages/opencode/src/tool/task.ts

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