Add CSV export option to `cortex export`

[KD-K2N2]

Description

Currently cortex export only outputs JSON. Add a --format csv flag for CSV export.

Expected behavior

$ cortex export --format csv --output memories.csv
✓ Exported 47 memories to memories.csv

$ head memories.csv
id,project,type,content,importance,tags,created_at
abc123,karnyx,decision,"Using Fastify adapter",8,"nestjs;architecture",2026-03-22T10:30:00Z

Implementation

• Add --format <json|csv> flag (default: json)
• CSV should include: id, project name, type, content, importance, tags (semicolon-separated), created_at
• Use simple string building — no need for a CSV library

Good first issue

Straightforward file I/O addition. Look at the existing export command in packages/cli/src/index.ts.