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/src/index.ts

@@ -262,12 +262,13 @@ export function apply(ctx: Context, config: Config): void {
     disposeTool = ctx.tools.register(defineTool({
       name: config.toolName ?? 'subagent',
       description: wording.description + (backgroundEnabled
-        // The return channel is a separately installed capability this package
-        // cannot observe, so this describes only this call's result.
+        // The completion notice is the continuation service's own behavior, not
+        // a separately installed capability, so this promise holds whenever the
+        // continuable background path is reachable at all.
         ? continuable
           ? ' Set `run_in_background: true` to start a background subagent that keeps its conversation:'
-          + ' you receive only its subagent id, never its result, and it works on its own. Use this for'
-          + ' work whose result you do not need returned by this call; `send_message` sends it more work.'
+          + ' this call returns only its subagent id, and the subagent works on its own from there. You'
+          + ' are told when it finishes, so never poll or wait on it; `send_message` sends it more work.'
           : ' Set `run_in_background: true` to return a task id; collect with `task_output` and stop with `task_kill`.'
         : ''),
       parameters: {
@@ -286,7 +287,7 @@ export function apply(ctx: Context, config: Config): void {
             type: 'boolean' as const,
             description: continuable
               ? 'Run as a background subagent that keeps its conversation and return only its subagent id. '
-              + 'This call never returns its result; send it more work with send_message.'
+              + 'This call does not wait for it; you are told when it finishes. Send it more work with send_message.'
               : 'Run as a background task and return its id; collect with task_output or stop with task_kill.',
           },
         } : {},