[codex] Enforce concept alias foreign keys

[yibeichan]

Summary

• enable SQLite foreign-key enforcement for writable vault DB connections
• make the concept alias test fixture use FK-backed concept slugs
• add regression coverage for nonexistent duplicate and canonical slugs

Root cause

SQLite does not enforce foreign-key constraints unless each writing connection enables PRAGMA foreign_keys = ON. addConceptAlias opened a writable schist.db connection without that pragma, so orphaned concept_aliases rows could be inserted silently.

Validation

• npm test -- --testPathPatterns=memory.test.ts
• npm test

Closes #196