Add semantic session negotiation

[kevinyang372]

Summary

Adds an additive, fail-closed semantic conversation session mode to the shared-session protocol while preserving the legacy full-terminal default and JSON compatibility.

The contract includes:

• explicit content-mode, schema-version, and execution-identity negotiation
• exact session/execution scoped cursors
• semantic ordered mutation events and typed resync reasons
• accepted-message context transport
• positive feature gating for new server-to-client variants
• legacy and semantic JSON compatibility fixtures

Compatibility

Older clients continue to omit the new fields and use full-terminal behavior. Semantic clients require an exact server echo and reject missing or mismatched mode, schema, execution identity, or cursor scope.

Validation

• cargo +1.88.0 fmt -- --check
• cargo +1.88.0 clippy --all-targets --all-features -- -D warnings
• cargo +1.88.0 nextest run --workspace --no-tests=warn (10 tests passed)
• git diff --check

Plan: https://staging.warp.dev/drive/notebook/BOjlgsw2BUunaAz7Hxzt0I
Conversation: https://staging.warp.dev/conversation/3c209bd3-23e2-402b-9c0c-52a0167a4167