feat(run-engine): add optimized message format to reduce Redis storage by ~80%

This implements a new V3 message format that eliminates separate message keys
for pending runs, encoding all necessary data directly in the sorted set member.

Key changes:
- Add messageEncoding module with encode/decode helpers for V3 format
- Add useOptimizedMessageFormat option to RunQueue for gradual rollout
- Update Lua scripts to handle both legacy and V3 formats (dual-read)
- Add V3-specific enqueue/nack Lua scripts (V3 write when enabled)
- Add comprehensive tests for encoding/decoding roundtrips

Migration strategy:
1. Deploy with useOptimizedMessageFormat=false (new code reads both formats)
2. Enable useOptimizedMessageFormat=true (new messages use V3)
3. Old messages drain naturally as they're processed (no backfill needed)

Storage reduction: ~88% per pending message (442 bytes -> 53 bytes)

https://claude.ai/code/session_01AyzQp6tbj7th5QRTCYjJR5

Author: claude