Droid: Add support for BYOK models

[rlgrpe]

Currently, the iPhone app only shows the models listed under “Available Models,” but not those listed under “Custom Models.” In droid exec --help, they are visible

[dnakov]

Reviewed current model selection code. Main now requests models per runtime and the picker groups/displays runtime-scoped models, but I do not see a Droid-specific path that extracts the separate Custom Models section from droid exec --help when model/list does not expose those entries. Leaving this open; the fix should be Rust-first in the runtime model refresh path, with a sample Droid help/model payload so we can add the custom models without hardcoding them in Swift/Kotlin.