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>

event-model/target/debug/.fingerprint/itoa-e940bf7c9c083c64/invoked.timestamp

@@ -0,0 +1 @@
+This file has an mtime of when this was started.

event-model/target/debug/.fingerprint/itoa-e940bf7c9c083c64/lib-itoa

@@ -0,0 +1 @@
+28d1a47e1679c2ee

event-model/target/debug/.fingerprint/itoa-e940bf7c9c083c64/lib-itoa.json

@@ -0,0 +1 @@
+{"rustc":2467252760697041086,"features":"[]","declared_features":"[\"no-panic\"]","target":18426369533666673425,"profile":15657897354478470176,"path":14942821222029544552,"deps":[],"local":[{"CheckDepInfo":{"dep_info":"debug/.fingerprint/itoa-e940bf7c9c083c64/dep-lib-itoa","checksum":false}}],"rustflags":[],"config":9396254390672932401,"compile_kind":0}