feat(memory): Memory Provider Abstraction

[acreeger]

Parent: #318

Requirements

1. Create MemoryProvider interface with store and query operations
2. Create MemoryService that wraps providers and handles configuration
3. Add memory.enabled (boolean) and memory.provider (string) to settings schema
4. Service must check memory.enabled before any operations
5. When memory is disabled, all operations are no-ops (no errors)
6. Service must gracefully handle missing/unavailable provider

Acceptance Criteria

• Settings schema updated with memory configuration options
• Provider interface defined for store/query
• Service returns cleanly when memory disabled
• Service handles provider errors without crashing