Hook modules can mark a result as withheld, and a matcher error is reworded

Hooks are pieces of code, often supplied by plugins, that Claude Code runs at set points. Two small changes affect hook modules:

• When a hook module returns a result marked as withheld, Claude Code now calls that module's own withheld handler, if it has one, before going on.
• An error about an invalid key in a hook's matcher, the rule deciding which events the hook responds to, now ends with "which a matcher does not keep" instead of "which no event has".