refactor(agent): hand the capture watchers a DeviceAccess

`gesture::spawn` and `keyboard::spawn` each took the four device-access
handles as separate parameters, and each watcher then carried them in two more
structs: its manager context, and the channels it hands every session
(`SessionChannels`, `KeyboardSessionChannels`), with the receiver lease
travelling beside them as a fifth argument through `reconcile` and
`drain_for_shutdown`.

Both take a `DeviceAccess` and keep it whole. The agent passes
`shared.device_access()` to the gesture watcher and `shared.keyboard_access()`
to the keyboard watcher, which is the one difference between them: which
capture slot they publish into.

The host-switch watcher is deliberately not part of this. Its
`SessionServices` holds three of the handles and a `ChannelPool`, and no
capture slot: it never publishes or reads one, and a `DeviceAccess` would hand
it a slot it has no business touching.

Guard: agent-device-access-owner flags a struct or a parameter list outside
hardware.rs and orchestrator.rs that names both `CaptureChannelSlot` and
`ChannelRegistry` (8 hits before the DeviceAccess consolidation, 6 on the
parent tree, 0 here).