Implements RedisStorage — a Redis-backed session.Storage that enables
session sharing across proxyrunner replicas for horizontal scaling
(R-VMCP-6). Supports standalone and Sentinel failover modes, optional
per-connection TLS (including separate master/sentinel configs), and
relies on Redis key TTLs for expiry rather than a background sweep.

Also activates the previously-unused serializeSession/deserializeSession
helpers in serialization.go by removing their nolint:unused annotations.

New files:
- pkg/transport/session/redis_config.go — RedisConfig, SentinelConfig,
  RedisTLSConfig types and default timeout constants
- pkg/transport/session/storage_redis.go — RedisStorage implementation
- pkg/transport/session/storage_redis_test.go — full test coverage via
  miniredis (constructor validation, round-trips for all session types,
  TTL refresh/expiry, key format, TLS config, Delete/DeleteExpired)

Closes: #266