Multi-tenant mode: --multi-tenant with per-user brain files

[omoshola-o]

Add --multi-tenant --data-dir /data/users/ flag to serve-http.

Behavior

• Header: X-User-ID: user123
• File mapping: /data/users/user123.amem
• Auto-creates user file on first request
• Returns error -32804 if X-User-ID header is missing in multi-tenant mode

Usage

agentic-memory-mcp serve-http \
  --multi-tenant \
  --data-dir /data/users/ \
  --port 8080 \
  --token "secret123"

Part of #1