#307 docs: add comprehensive Murmur vs Agent Relay protocol comparison

Summary

Adds detailed competitive analysis comparing Murmur's end-to-end encrypted cross-network messaging with Agent Relay's low-latency local IPC.

Key Findings

Complementary Systems:

• Murmur: 100-500ms latency, E2E encrypted (Signal Protocol), offline-first, cross-network
• Agent Relay: <5ms latency, local IPC, real-time coordination, synchronous messaging

Improvement Opportunities for Agent Relay

High Priority

1. ✅ Message persistence - Optional SQLite storage for critical messages
2. ✅ Backpressure implementation - Bounded queues, BUSY responses
3. ✅ Session token security - Expiration, rotation, replay protection

Medium Priority

1. ✅ Error code taxonomy - Complete enumeration beyond just "STALE"
2. ✅ Attachment support - Reference-based with chunked upload
3. ✅ TTL + auto-cleanup - Prevent unbounded memory growth

Content

The comparison document covers:

• Transport layer differences (HTTPS+SSE vs Unix Domain Sockets)
• Message format and structure (3-layer encryption vs single envelope)
• Security models (cryptographic vs trust-based)
• Delivery guarantees (store-and-forward vs online-only)
• Session management (stateless vs stateful)
• Cryptography deep dive (X3DH, Double Ratchet, etc.)
• Synchronous messaging (Agent Relay's unique feature)
• Integration scenarios (hybrid architecture, gateway pattern)

Test Plan

Documentation change only - no code changes.

🤖 Generated with Claude Code

---