fix(session): notify parent when subagents finish

[opencode-agent]

Summary

• emit a synthetic task result into the parent when any child session finishes
• suppress that parent notification for foreground task calls while their job is still owned by the tool result
• remove task-tool-specific background result injection so background completion flows through child-session completion

Fixes#35066

Tests

• bun test test/session/prompt.test.ts test/tool/task.test.ts
• bun typecheck

[github-actions]

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