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/Cargo.lock

@@ -26,6 +26,7 @@ dependencies = [
  "axum-server",
  "base64 0.23.1",
  "clap",
+ "event-model",
  "libc",
  "rand",
  "ron",
@@ -543,6 +544,14 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "event-model"
+version = "0.1.0"
+dependencies = [
+ "serde",
+ "serde_json",
+]
+
 [[package]]
 name = "fastrand"
 version = "2.5.0"