fix(task): recover stale delegated children after restart (#1210)

* fix(task): recover stale delegated children after restart

* fix(task): make delegated recovery durable

* test(e2e): verify persistence across VS Code restart

* test(e2e): verify persistence across VS Code restart

* fix(task-persistence): harden delegation repair recovery

* test(e2e): stabilize restart persistence smoke test

* fix(task): serialize concurrent history restoration

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Co-authored-by: Roomote <roomote@roomote.dev>