## Why
The process-owned code mode implementation needs an explicit, bounded
wire contract before either side depends on it. Keeping framing and
message semantics in `codex-code-mode-protocol` gives the client and
sidecar one shared source of truth and makes compatibility failures
detectable during connection setup.
## What changed
- adds a versioned client/host handshake with required and optional
capabilities
- defines operation requests and responses for session lifecycle and
cell control
- defines reverse delegate request, response, cancellation, and
cell-closure messages
- adds a four-byte little-endian length-prefixed JSON codec with a hard
frame cap
- rejects malformed frames, unknown fields, invalid identifiers, and
unsupported protocol states
- locks the wire representation down with explicit JSON round-trip tests
## Testing
- `just test -p codex-code-mode-protocol`
## Stack
Part 1 of 6. Followed by
[#29805](https://github.com/openai/codex/pull/29805).
## Summary
- add validated protocol-version, capability, and session identifier
types
- define explicit `ClientToHost` and `HostToClient` JSON envelopes for
connection negotiation and session open/close acknowledgements
- reject invalid states and unknown fields during decoding, with
explicit wire-format and round-trip coverage
## Why
This establishes the transport-neutral encoding shape needed to build
and test the new code-mode host incrementally. Cell, tool callback, and
failure-domain messages are intentionally deferred until their actors
and behavior tests establish the required semantics.
This is additive protocol scaffolding and does not change the current
production code-mode implementation.
## Validation
