feat: add RedisMemoryStore backend

[zeroknowledge0x]

Summary

Implements a Redis-backed MemoryStore for production deployments, as requested in #2.

Changes

• src/memmark/store/redis_store.py: New RedisMemoryStore class extending MemoryStore
  • Connection pooling via redis-py
  • Configurable key prefix and TTL
  • Graceful error handling on connection failures
  • clear() and size() convenience methods
• src/memmark/store/__init__.py: Converted store module to package (moved original store.py content here, exports RedisMemoryStore)
• pyproject.toml: Added redis optional dependency group, added fakeredis to dev deps
• tests/test_redis_store.py: 19 tests covering read/write/append/clear/size, TTL, corrupted data, connection errors, and initialization

Acceptance Criteria

• RedisMemoryStore passes all MemoryStore interface tests
• Works with default Redis config (localhost:6379)
• Graceful error handling on connection failure
• All tests pass (294 total) and lint is clean

Usage

from memmark.store import RedisMemoryStore

store = RedisMemoryStore(url="redis://localhost:6379/0", ttl=3600)
store.write([{"id": "m1", "content": "hello"}])
memories = store.read()

Install with: pip install memmark-agent[redis]

Closes #2