Device hosts: support a configured simulator device set (and AVD home)

[gjw4118]

Problem

T3's device host lists iOS simulators with xcrun simctl list devices -j, which reads only the default device set.

• A simulator created in a custom set (xcrun simctl --set <dir> create …) boots fine, but never appears in device_list or the Device panel, and device_open refuses it ("No device … on ''").
• On Android, a stopped AVD under a custom ANDROID_AVD_HOME isn't listed, so it can't be booted from the panel. It can be driven once it's running.

Why it matters

Build hosts often have a small internal disk and a large external SSD. Here it's a Mac mini with a 228 GB internal disk and a 1 TB SSD, used as a T3 device host. Simulators are 3–14 GB each, so keeping them in the default set (~/Library/Developer/CoreSimulator/Devices) fills the internal disk. Symlinking that folder isn't a safe option.

Simlock pools already keep devices on an external volume, and T3's Simlock integration works well for that. A plain local or SSH device host can't do the same.

Proposal

A per-device-host setting for:

• an iOS simulator device-set path, used when listing, booting and streaming, and passed through to agent-device. The bundled agent-device already supports AGENT_DEVICE_IOS_SIMULATOR_DEVICE_SET;
• an Android AVD home, so stopped AVDs there are listed and bootable.

Tested on: T3 0.1.6-nightly.20261030 and 0.1.7-nightly.20261101, Xcode 27.1, iOS 27.0 and 27.1 runtimes, macOS 27 on Apple silicon.