feat(tasks): wake an idle owner when a background task completes

Completion notices went through agent.inject(), which never reserves a
driver, so a task settling after its turn closed left the notice parked
until unrelated input woke the agent — while the same prompt told the
model not to poll for it.

An unreported completion now picks its lane from the owner's state: a
busy owner is injected as before, an idle owner is woken with
followup(). This adopts the delivery rule the subagent continuation
manager already ships. maxConsecutiveWakes bounds the self-exciting
chain and is reset by user-authored input; completionDelivery: quiet
restores the old lane for deterministic transcripts.

Teardown cancellation now claims the terminal report the way kill()
already does, so an owner being destroyed is never woken, and settle()
announces completion last so a reporter that opens a turn synchronously
sees a committed record.

docs/subsystems/tasks.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write docs/subsystems/tasks.md
-tasks.md: 2798eb2963e83d42321a343d3c0a6a2bb658be41
-tasks.zh.md: 8d4539514cb4bc654dd86031d51417c4f79f8982
+tasks.md: 37807bc446f607c3e1635a49432670e86760f2a4
+tasks.zh.md: 39014acb13f6051431a40ae084c1e03298ae8993