Part of #1.

What changed

• add retention filter helpers plus Qdrant scroll support for cleanup queries
• prune stale low-signal episodic memories after consolidation
• prune long-expired superseded semantic facts after consolidation
• document the retention step and cover it with focused memory tests

Why

The earlier memory PRs improved ranking, contradiction handling, and startup context, but stale memories were still only being down-ranked at retrieval time. This change adds the forgetting side of the system so low-value episodes and obsolete facts can age out during consolidation.

Validation

• bun test src/memory/__tests__/episodic-retention.test.ts src/memory/__tests__/semantic-retention.test.ts src/memory/__tests__/consolidation.test.ts src/memory/__tests__/qdrant-client.test.ts
• bun run typecheck
• bun run lint
• bun test