Moving `web-agent-presets` to the built lane took its coverage with it, and the
per-file gate named what it had been the only reach for: the session header's
`agentPreset` type check, and the preset subtree's `write()` override. Neither
belonged to a real-composition boot — a header validation case and a subtree
behaviour are package-level facts.
The `write()` test states why the override exists: the inherited method
persists the tree whenever the Loader thinks the config moved, and disposing an
agent disposes its subtree, so inheriting it truncates the shipped preset to
`[]` the first time a session ends.
Yichen Jiang
7233d40fc9