Headless context-inheriting subagents

Problem

An explicit subagent_type: "fork" request is currently honored only when Config.isInteractive() is true. Headless callers such as qwen --prompt, the TypeScript SDK, and CI runners silently execute a fresh general-purpose subagent instead. The requested and effective context modes therefore differ, and the child does not receive the parent conversation.

Design

Fork availability is independent of the presentation surface. A top-level fork request always uses the existing fork construction path, which copies the parent's history and cache-safe generation configuration.

Headless forks run through the existing background-agent registry even when run_in_background is omitted or false. Forks are detached by definition, and the registry gives non-interactive callers the lifecycle they need:

• one-shot headless execution waits for the fork to finish;
• stream consumers receive task_started and terminal task notifications;
• the effective subagent_type: "fork" is recorded in events, metadata, and subagent telemetry;
• permission requests that cannot be shown in a non-interactive session are denied by the existing background-agent policy instead of hanging.

Interactive fork behavior remains unchanged.

A fork request from a nested subagent is still unsupported, but it now fails with an explicit tool error instead of silently running a fresh general-purpose subagent.

Scope

This change reuses the current full-history fork behavior. It does not add partial history selection such as fork_turns; that can be introduced separately without blocking correct headless inheritance.

Verification

• Core dispatch tests cover interactive forks, headless forks, forced background lifecycle, inherited history construction, permission behavior, and explicit nested-fork rejection.
• The non-interactive CLI test covers the SDK-facing task_started event and verifies that it exposes subagent_type: "fork".
• The desktop SDK adapter test verifies that the runtime's background result takes precedence over a caller-provided run_in_background: false.
• An end-to-end qwen --prompt --output-format stream-json check uses a parent marker that is absent from the fork directive and verifies that the child can still recover it from inherited history.