[MCP] println!/eprintln! 可能破坏 MCP JSON-RPC 协议

[TrueNine]

println!/eprintln! to stdout can corrupt MCP JSON-RPC protocol communication. MCP uses stdout for JSON-RPC messages.

File: mcp/src/commands/package.rs:45,50

[TrueNine]

Fixed. MCP stdio mode no longer emits startup logger spans/messages to stdout, and the hidden assemble-npm packaging command now routes human-readable status/errors through stderr only.\n\nAdded regression coverage for #225, including a binary-level test that asserts assemble-npm failures leave stdout empty so JSON-RPC framing remains safe.