feat(subagent): deliver continuable child settlement to parents

A continuable child that stopped without reporting — an error, a token
ceiling, cancellation, teardown — left its parent nothing to act on.
The continuation manager now delivers an unconditional settlement
notice to the durable direct parent before releasing ownership, folding
consumed work (foldConsumedWork supersedes findLastMessageTurnEnd) so a
claimed-but-unrun prompt reads as aborted rather than completed, waking
an idle parent, steering a busy one, and never waking a closing tree.

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, and the child does not report back — its transcript by that id is the source of its output, and the optional global `send_message` tool sends it more work. 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).
+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).
 
 `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).
 
@@ -37,7 +37,7 @@ Foreground and background calls are exclusive. Children may share the parent's w
 
 #### 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 and returns its subagent id, 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`, 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`.
 
 #### Token effect
 
@@ -65,11 +65,11 @@ Append-only; newly visible content follows the reusable request prefix and does
 
 #### What the model sees
 
-Start returns exactly `started subagent <childId>` in configured continuable mode, or `started background subagent task <id>` in configured one-shot mode. In one-shot mode the generic task surface provides later status, final output, cancellation responses, and notices. In continuable mode the child does not report back; an independently loaded `send_message` tool delivers follow-ups, and the child's transcript by its id is the source of its output.
+Start returns exactly `started subagent <childId>` in configured continuable mode, or `started background subagent task <id>` in configured one-shot mode. In one-shot mode the generic task surface provides later status, final output, cancellation responses, and notices. In continuable mode this tool returns no result of its own; the child's settlement reaches the parent as a [service-owned notice](../subagent/README.md#settlement-notice), an independently loaded `send_message` tool delivers follow-ups, and the child's transcript by its id is the source of its detailed output.
 
 #### Token effect
 
-The acknowledgement is retained; a one-shot final output enters parent history only when collected or injected, while a continuable child's output never returns through this tool.
+The acknowledgement is retained; a one-shot final output enters parent history only when collected or injected, while a continuable child's output never returns through this tool — its settlement notice arrives independently of any tool result.
 
 #### 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.
+- **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.
 - **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.