feat: initialize foundation and hybrid gRPC core loop

[MrBT-nano]

📝 Description

This PR initializes the Rust workspace and sets up the foundation for the vtuber-brain director loop using a Hybrid gRPC approach. It implements the initial contracts and service for receiving context and emitting directives.

Key changes:

• Initialized Cargo workspace with brain-core, brain-proto, and brain-grpc crates.
• Defined gRPC contracts for BrainService (Input) and VoiceService (Output).
• Implemented gRPC server for PushContext and a test binary.
• Adopted Ecosystem Interaction Protocol in GEMINI.md and CLAUDE.md.

🔗 Related Issues

Fixes #1

🛠️ Type of Change

• 🚀 New feature
• 🐛 Bug fix
• 📄 Documentation update
• 🧪 Testing
• 🔨 Refactoring
• ⚙️ CI/CD

✅ Checklist

• Tests passed locally
• Documentation updated
• Code follows project style guidelines
• Linked to the appropriate Issue
• cargo fmt and cargo clippy pass

🏷️ Metadata

• Assignee: @MrBT-nano
• Labels: feat, foundation, grpc

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