AGENTS.md: the phone app and the planned desktop app share widgets and styling; only screen layout differs

Iris, 2026-09-07. The second central design point beside the driver
rule, so a platform crate growing a widget or a colour reads as a
defect to move. docs/RUST.md carries the detail.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

AGENTS.md

@@ -19,6 +19,21 @@ child process, translated into one common event model.** A new session type
 is a new driver — never a session-type branch in shared code (routes,
 transcript, app screens).
 
+The second one, for the Rust port on the `rustify` branch: **the phone app
+and a planned desktop app share almost all of their code.** Screens, widgets,
+folding, paging, config and the network client live in the shared crates
+(`iris`, `client-core`, `transcript-ui`, `tabs-ui`); `android-app` and
+`desktop-app` are thin entry points that own only what the platform forces
+(JNI and the IME on one side, winit and argv on the other). The two
+*layouts* will differ, to suit a phone's screen and a finger against a
+desktop's screen and a mouse -- but the widgets a layout is made of (a
+button, a text field, a list, a card) and the styling (colours, spacing,
+type) are one implementation with no per-platform copy. Anything that could
+work on both goes in a shared crate the first time it is written, and a
+platform crate growing a widget or a colour is a defect to move, not a
+convenience to keep. Iris said this on 2026-09-07; docs/RUST.md carries the
+details.
+
 ## Layout
 
 Mirrors `../dev-updater` deliberately: same stack (axum 0.8 +