fix: match archived changes by exact slug

[fengjikui]

Summary

• avoid resolving archived changes by suffix-only directory matches
• keep legacy non-dated archive names supported via exact slug matches
• add regression coverage for user-auth not matching 2026-01-01-add-user-auth

Root cause

Archived lookup accepted any archive directory whose name ended with -${slug}. That means a lookup for user-auth could incorrectly match an unrelated archived change such as 2026-01-01-add-user-auth, because the directory has the same suffix.

Validation

• npm test -- --run test/reference-scripts.test.ts
• npm test
• npm run typecheck
• npm run build
• git diff --check