feat: add agentId namespace support for memory isolation

[daniel-harperdb]

Summary

Adds agentId as an indexed field on the Memory table, enabling multi-agent memory isolation.

• Adds agentId: String @indexed to schema.graphql
• MemorySearch filters by agentId when provided
• Enables multiple agents to share a Cortex instance without cross-contamination

Test plan

• Unit tests included (test/namespace.test.js)
• Manual test: store memories with different agentIds, verify search isolation