Prune commentary and stale Rust port notes

AGENTS.md

@@ -196,10 +196,9 @@ guaranteed to have.
   own suite, which is slower and not about this product. Each workspace
   also gets `cargo clippy --all-targets` and `cargo fmt`. The build stays
   warning-clean and rustfmt-clean at the defaults — there is no
-  `rustfmt.toml` and there should not be one. `app-rust/` and `iris/` are
-  pinned to the same dated nightly (`rust-toolchain.toml`, one copy each,
-  because a pin applies per directory); `server/` and `event-model/` are
-  stable.
+  `rustfmt.toml` and there should not be one. `app-rust/`, `iris/`, and the
+  UI profiling rig use the rolling nightly channel through per-directory
+  `rust-toolchain.toml` files; `server/` and `event-model/` are stable.
 - **App**: from `app/`,
   `. ./android-env.sh && ./gradlew :androidApp:ktfmtFormat
   :androidApp:compileDebugKotlin :androidApp:lintDebug