acp: implement stdio JSON-RPC transport

[bug-ops]

Parent: #762

Task

Implement the stdio transport layer: read newline-delimited JSON-RPC from stdin, write to stdout.

Acceptance criteria

• transport.rs — StdioTransport struct with async read_message() and write_message() methods
• Newline-delimited framing per ACP spec (no embedded newlines in messages)
• UTF-8 encoding enforcement
• Diagnostic logging to stderr via tracing
• Graceful shutdown on stdin close
• Unit tests with mock stdin/stdout using tokio::io::duplex

[bug-ops]

Implemented in #774: serve_stdio() with AgentSideConnection over tokio-compat stdio streams