🔮 P4: Switch stdio handshake to google-protobuf gem for future-proofing

Context

The localharness binary uses a stdio handshake with 2 tiny protobuf messages:

• InputConfig (5 fields: storage_directory, port, bind_address, client_info, env)
• OutputConfig (2 fields: port, api_key)

All WebSocket messages are JSON-serialized (not binary protobuf), so no gem needed there.

Current Approach (v0.4.0)

Hand-rolled binary encoding for these 2 messages using Array#pack (~30 lines).
Zero additional dependencies. Functionally identical.

Future-Proof Approach (this issue)

Add google-protobuf gem (~3.6 MB) and use compiled .proto files:

• content.proto → content_pb.rb
• localharness.proto → localharness_pb.rb

Proto files already compile cleanly with protoc --ruby_out.

Why Later

• Only 2 messages need binary encoding today
• The gem adds 3.6 MB + native C extension
• JSON handles all WebSocket communication
• If upstream proto schema changes significantly, we upgrade then

Proto compilation command (for reference)

protoc --ruby_out=lib/antigravity/proto/ \
  --proto_path=/tmp/antigravity-sdk-python/ \
  /tmp/antigravity-sdk-python/google/antigravity/proto/content.proto \
  /tmp/antigravity-sdk-python/google/antigravity/proto/localharness.proto

Labels

Priority: P4 (nice to have)
Type: Feature Request

Start by locating the stdio handshake implementation that currently uses Array#pack, then review content.proto and localharness.proto and the referenced protoc command. Add the generated Ruby protobuf files and dependency, and verify that the two handshake messages use them while WebSocket messages remain JSON-serialized.