docs: Add replay protection documentation with nonce/timestamp examples

[nifanpinc]

Summary

This PR adds comprehensive replay protection documentation to address #103.

Changes

• Added Replay Protection section to README.md
• Included nonce + timestamp validation pattern with code examples
• Added server-side validation code in Python
• Documented error codes (TIMESTAMP_STALE, NONCE_REUSED, SIGNATURE_INVALID)
• Added idempotency guidance for message retries

Implementation Pattern

# Client-side: Include nonce + timestamp
envelope = BeaconEnvelope(
    kind="message",
    payload={"text": "Hello"},
    nonce=generate_monotonic_nonce(),
    timestamp=int(time.time())
)

# Server-side: Validate
if abs(time.time() - envelope.timestamp) > 30:
    raise ReplayError("TIMESTAMP_STALE")
if nonce_store.has_seen(envelope.nonce):
    raise ReplayError("NONCE_REUSED")

Checklist

• Added clear code examples
• Documented all error codes with resolutions
• Included idempotency guidance
• Referenced existing docs/SECURITY.md

Related

Closes #103