Extract the event model into its own crate, shared with client-core

RUST.md's recommendation item 1 starts here: Event, QuestionOption,
SessionStatus, ImageRef, AttachmentRef, SeqEvent, context_tokens and
context_after move to a new event-model crate so a future Rust client
shares one definition with server/ instead of Events.kt's hand-kept
mirror. session/driver.rs and session/transcript.rs re-export
everything they used to define, so nothing downstream of either
module changed.

cargo test (127 passed), clippy --all-targets and fmt clean in both
server/ and event-model/.

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

server/src/session/transcript.rs

@@ -13,20 +13,14 @@ use std::os::unix::fs::OpenOptionsExt;
 use std::path::Path;
 
 use anyhow::{Context, Result};
-use serde::{Deserialize, Serialize};
+use serde::Deserialize;
 
 use super::driver::{Event, SessionStatus, context_after};
 
-/// One transcript line: an [`Event`] plus its position and time. The event
-/// is flattened so the wire shape stays one flat object.
-#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
-pub struct SeqEvent {
-    pub seq: u64,
-    /// Epoch seconds.
-    pub ts: f64,
-    #[serde(flatten)]
-    pub event: Event,
-}
+// `SeqEvent` moved to `event-model` on 2026-09-04 along with the rest of the
+// event model, so `client-core` can read the same wire shape; re-exported
+// here since every caller in this crate reaches it through this module.
+pub use event_model::SeqEvent;
 
 pub struct Transcript {
     file: File,