Device panel: a physical device is lent to one thread at a time; simulators per thread #163

Problem

The Device panel opens an iOS Simulator or Android Emulator for a thread (device_open, pinned agent-device CLI). A device host can also have a physical phone attached over USB. A simulator can be cloned per thread, but a physical device is one screen, one input and one installed app state.

If two threads open the same physical device, they step on each other: one installs a build while the other taps through a flow. Nothing in the tools prevents that today, so the only guard would be agents agreeing in chat. We are removing that kind of coordination (threads must work independently).

Proposal

• A physical device is lent to one thread at a time. device_open on a device held by another thread fails with a clear reason (holder thread, since when) instead of attaching.
• The lease ends when the thread closes the device or ends; a stale lease expires.
• device_list shows the holder.
• Simulators and emulators stay per thread. Where a host can, device_open without a deviceId gives the thread its own clone, so parallel threads never share one.

Acceptance

• Two threads open the same physical device: the second gets a refusal naming the holder; after the first closes it, the second succeeds.
• Two threads open "an iOS simulator": each gets its own device.

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