event-model, client-core, transcript-ui: carry main's LimitReached event

The merge that brought main into rustify added Event::LimitReached to the
server's drivers, but on this branch the enum lives in event-model, which
the merge left without it, so ai-server (and ui-sandbox.sh) did not build.
Definition copied from main's driver.rs; the fold mirrors TranscriptItems.kt's
LimitNote; the iris row shows the epoch until P1 brings a time formatter.

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

event-model/src/lib.rs

@@ -305,6 +305,25 @@ pub enum Event {
     /// it, which is why this is written down rather than left to be inferred
     /// from a second example that does not exist.
     Cleared,
+    /// The account behind this session has no quota left, so the turn stopped
+    /// without finishing.
+    ///
+    /// Its own event rather than an [`Event::Error`] carrying the dialect's
+    /// sentence, because two things act on it that cannot read English: the
+    /// transcript draws it as a state the session is in rather than as a
+    /// failure of something it did, and `crate::resume` schedules the message
+    /// that picks the work back up. Recognising it belongs to the driver, which
+    /// is the only layer that knows its dialect's wording -- above here nothing
+    /// matches on strings.
+    ///
+    /// `resets_at` is epoch seconds, and `None` is a real state: the dialect
+    /// said the limit was hit without saying when it lifts. Nothing here
+    /// invents one -- what the wait is actually decided against is the usage
+    /// endpoint, and this is the hint that starts the waiting.
+    LimitReached {
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        resets_at: Option<f64>,
+    },
     Error {
         message: String,
     },