Three-layer companion to the salvaged CLI drain-ownership fix (#64240):
1. restore_undelivered_completions stamps restored=True (in-memory only)
on every durable completion re-enqueued at process start.
2. drain_notifications' legacy unfiltered branch re-queues restored
events instead of consuming them — a fresh process can no longer
adopt a dead session's delegation results (#64484). Same-process
keyless events keep the legacy behavior.
3. delegate_tool's async dispatch now falls back to the parent agent's
durable session_id when the approval-context key resolves empty (the
CLI case), so the CLI's new positive-ownership drain can actually
claim its own completions instead of failing closed on ''.
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