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/quote-f95d225106368b84/lib-quote.json

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