feat(cli): support background subagents

[Githubguy132010]

Context

Enable interactive CLI agents to delegate independent work to background subagents so the main agent can continue while child sessions run. This implements the v1 scope from #9611: model-initiated backgrounding, user notification, and parent-agent notification when the subagent finishes.

Implementation

• Extended the task tool with background?: boolean while preserving existing foreground behavior by default.
• Reused the existing child-session subagent model, storing metadata.background alongside metadata.sessionId.
• Added a background prompt path that immediately returns to the parent, runs the child session in the session prompt runtime, publishes a completion event, and queues a synthetic parent-session message with the result or error.
• Updated the TUI task renderer to show background task state from child session status and added a completion toast for background subagent close events.
• Added task-tool test coverage for immediate-return background behavior and updated test stubs for the new prompt op.

Screenshots

before	after
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How to Test

• Run bun test test/tool/task.test.ts test/kilocode/tool-task-model.test.ts from packages/opencode/.
• Run bun run typecheck from packages/opencode/.
• In the TUI, ask the main agent to start an independent investigation in a background subagent and continue with another task.
• Confirm the task tool returns immediately with a task_id, the child session continues running, and a toast plus synthetic parent-session update appear when it finishes.

Get in Touch

Open to feedback directly on this PR.

Fixes #9611