feat(subagent): default continuable delegation to background

packages/subagent/tool-subagent/README.md

@@ -10,7 +10,7 @@ Each plugin instance binds one `provider` to one `toolName`; the model receives
 
 A foreground call passes the execution signal through startup and execution, awaits `run.result`, and always awaits `run.dispose()` before returning. Only `completed` returns the canonical `{ kind: 'foreground', runId, output: JsonValue[] }`, rendered as the same final text; abort, refusal, token limit, and other failures become errored tool results whose message appends the child's preserved partial text (the `SubagentResult.output` selection) after the stop-reason headline, so a truncated answer is never reported as success yet never silently lost. If result collection and disposal both reject, the errored result preserves both diagnostics.
 
-With `run_in_background: true`, `backgroundMode` selects the route. `one-shot` registers a plain parent-owned Task and returns canonical `{ kind: 'background', taskId }`, rendered as `started background subagent task <id>`, even when the provider supports continuable children; generic task tools own its later status, collection, cancellation, and notices. `continuable` requires a provider with the `prepareContinuable` capability, calls `ctx.subagents.startContinuable()`, and returns `{ kind: 'continuable', subagentId }`, rendered as `started subagent <childId>`. The continuable route resolves at inbox acceptance: the child owns its own turns from there, so this call neither waits for nor collects a result. The child's transcript by that id remains the source of its detailed output, and the optional global `send_message` tool sends it more work. The parent is not left guessing when to look, though: the continuation service delivers one settlement notice to it whenever a continuable child's Activation ends, which is why the schema tells the model it will be told and must not poll. Starting continuable work does not require `send_message` to be loaded. See the [background subagent Agent Note](../../../.agents/notes/implemented/feature/2026-07-08-background-subagent-tasks.md), the [continuable subagents Agent Note](../../../.agents/notes/implemented/feature/2026-07-28-continuable-subagent-conversations.md), and the [merged-service Agent Note](../../../.agents/notes/implemented/simplification/2026-07-26-merge-subagent-control-service.md).
+`backgroundMode` selects both the background route and the omitted `run_in_background` default. `one-shot` waits in the foreground by default; an explicit `true` registers a plain parent-owned Task and returns canonical `{ kind: 'background', taskId }`, rendered as `started background subagent task <id>`, even when the provider supports continuable children. Generic task tools own its later status, collection, cancellation, and notices. `continuable` runs in the background when the argument is omitted or `true`; an explicit `false` waits for the result in the foreground. Its background route requires a provider with the `prepareContinuable` capability, calls `ctx.subagents.startContinuable()`, and returns `{ kind: 'continuable', subagentId }`, rendered as `started subagent <childId>`. The route resolves at inbox acceptance: the child owns its own turns from there, so this call neither waits for nor collects a result. The child's transcript by that id remains the source of its detailed output, and the optional global `send_message` tool sends it more work. The continuation service delivers one settlement notice whenever the child's Activation ends, containing its outcome and any final assistant message independently of `report`. Starting continuable work does not require `send_message` to be loaded. See the [background subagent Agent Note](../../../.agents/notes/implemented/feature/2026-07-08-background-subagent-tasks.md), the [continuable subagents Agent Note](../../../.agents/notes/implemented/feature/2026-07-28-continuable-subagent-conversations.md), and the [background-first delegation Agent Note](../../../.agents/notes/implemented/feature/2026-08-11-background-first-continuable-delegation.md).
 
 `toolFilter` changes the child's global tool layer but is not a parent-derived authority ceiling. See the [agent-scope security non-goal](../../../.agents/notes/implemented/architecture/2026-07-08-agent-scope-contexts.md#security-and-authority-are-non-goals).
 
@@ -21,7 +21,7 @@ With `run_in_background: true`, `backgroundMode` selects the route. `one-shot` r
 | `provider` (required) | Provider name (`spawn`, `fork`, `acp`, ...). |
 | `toolName` | Model-facing name, default `subagent`; distinct for every loaded instance. |
 | `enableRunInBackground` | Exposes background mode, default `true`; disabling also rejects forced background calls. |
-| `backgroundMode` | Background lifecycle policy, default `one-shot`. `continuable` requires the provider's `prepareContinuable` capability and returns a durable child id; it does not require the follow-up tool. |
+| `backgroundMode` | Background lifecycle policy, default `one-shot`. `one-shot` defaults calls to foreground; `continuable` defaults them to background, requires the provider's `prepareContinuable` capability, and returns a durable child id without requiring the follow-up tool. |
 | `agentOptions` | Provider-specific child `provider`, `model`, and positive `maxTokens`; the in-process provider treats explicit values as overrides of inherited parent options. |
 | `persona` | Per-child persona; requires provider `persona` capability. |
 | `toolFilter` | Per-child global-tool restriction; requires `toolFilter` capability. |
@@ -37,11 +37,11 @@ Foreground and background calls are concurrency-safe: sibling delegations in one
 
 #### What the model sees
 
-The generated default [`subagent` schema](../../../docs/tool-catalog.md#deepseek-aidsh-tool-subagent) under this instance's configured name while its provider exists. Provider context inheritance changes the tool and prompt descriptions; enabled background mode adds `run_in_background`, and continuable mode describes starting a background subagent that keeps its conversation, returns its subagent id, and reports its own completion — so the model is told never to poll or wait on it — while one-shot mode describes a background task id collected with `task_output` and stopped with `task_kill`.
+The generated default [`subagent` schema](../../../docs/tool-catalog.md#deepseek-aidsh-tool-subagent) under this instance's configured name while its provider exists. Provider context inheritance changes the tool and prompt descriptions. Enabled background mode adds `run_in_background`: continuable mode documents its `true` default, runtime settlement notice, and explicit foreground override, while one-shot mode documents its `false` default and the task id collected with `task_output` or stopped with `task_kill`. A `tool:<toolName>` system-prompt section tells the model to start independent continuable delegations together, keep working while they run, and choose foreground only when its next action depends on the result.
 
 #### Token effect
 
-Fixed schema cost per parent request; each provider instance adds one schema.
+Fixed schema cost per parent request; each provider instance adds one schema, and each continuable instance adds one short system-prompt section.
 
 #### KV Cache effect
 
@@ -77,6 +77,6 @@ Append-only; newly visible content follows the reusable request prefix and does
 
 ## Known Limitations and Deferred Work
 
-- **Background runs expose no result through this tool** — a one-shot task's final output is collected through the generic task surface, and a continuable child's output stays in its own session, read by its subagent id. The settlement notice states how that child ended and carries its closing message, but it is not this call's return value and cannot be awaited here.
+- **Background runs expose no result through this tool** — a one-shot task's final output is collected through the generic task surface, and a continuable child's output stays in its own session, read by its subagent id. The settlement notice states how that child ended and carries any final assistant message, but it is not this call's return value and cannot be awaited here.
 - **Duplicate names across waiting instances are detected late** (`TODO(subagent-dup-toolname)`) — preventing provider-registration rollback requires a registry of intended names.
 - **Child policy is fixed per instance** — another model, persona, tool filter, or depth cap requires another distinctly named tool.