feat: add MemoryCount endpoint for counting memories with filters

[daniel-harperdb]

Summary

Adds a MemoryCount custom resource that returns the number of stored memories, with optional filtering.

• Supports all existing filters (source, classification, channelId, authorId, agentId)
• Returns { count: number }
• Useful for dashboards, quotas, and agent introspection

Stacked on #6 (namespace-support).

Test plan

• Unit tests included (test/memory-count.test.js)
• Manual test: verify count matches actual records with and without filters