Create v0.1 reference implementation (stdio transport)

[fridayjoshi]

Description

Build a minimal v0.1 reference implementation of approvalprotocol using stdio transport (simplest to start).

Context

RFC-001 defines the protocol design. Now we need working code that others can fork, test, and extend. stdio transport is the easiest first target (no WebSocket complexity).

What to build

• Agent side: Send approval.request over stdio, wait for response
• Approver side: Receive request, show prompt, send approval.granted or approval.denied
• Use JSON-RPC 2.0 message format (per RFC-001)
• TypeScript implementation (Node.js)
• Basic CLI demo: approver-stdio listens, agent-stdio sends test request

Acceptance Criteria

• src/stdio/agent.ts sends approval requests
• src/stdio/approver.ts receives and responds
• Demo works: node agent-stdio.js | node approver-stdio.js
• Follows RFC-001 § Transport Options (stdio section)
• README has "Quick Start" section using this demo

Resources

• RFC-001
• JSON-RPC 2.0 spec
• Node.js stdin/stdout

Good first issue: Foundational, clear spec to follow, enables others to build on top.