feat: add BrainService and PushContext definitions

[MrBT-nano]

Requirement

We need to formalize the BrainService interface as part of the canonical contracts. This will allow vtuber-api to send context to vtuber-brain using a shared typed boundary.

Proposed Changes

Add proto/echo/vtuber/brain/v1/brain.proto with:

syntax = "proto3";
package echo.vtuber.brain.v1;

service BrainService {
  rpc PushContext (PushContextRequest) returns (PushContextResponse);
}

message PushContextRequest {
  string session_id = 1;
  string user_id = 2;
  string message = 3;
  map<string, string> metadata = 4;
}

message PushContextResponse {
  bool accepted = 1;
  string request_id = 2;
}

This matches the standard established in vtuber-brain and follows the Canonical Protobuf Standard.