FOB-boundary dispatch for brain-proposed tool calls (ADR-0007: 'rutster
mediates both the provider call-control API and the brain tap, so the
brain never holds the wire'). Slice-3's only wired tool is hangup
(fires AppState::close → existing slice-2 teardown); other tool names
reply not_implemented.
TDD: 5 tests (dispatch known + unknown tool, catalog listing, hangup
schema shape, hangup_call fires AppState::close).
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