feat: expose arka primitives over MCP

[Genmin]

Summary

• add arka::mcp with an object-safe ArkaTool trait and a minimal stdio JSON-RPC server for initialize, tools/list, and tools/call
• add MCP tools for native balance reads, swap quote/build metadata, and AgentAccount deposit/balance operations
• add the arka-mcp-server binary and README wiring for Claude Desktop
• cover the server with no-network unit tests plus a stdio integration test that spawns the binary and drives MCP requests

Addresses #11.

Validation

• cargo fmt --check
• cargo build --bin arka-mcp-server
• cargo test --lib
• cargo test --test mcp_stdio
• cargo test --all-targets
• git diff --check