fix: prevent path traversal in memory_read daily date

[jayzeng]

Summary

• validate memory_read daily date input with strict YYYY-MM-DD format + calendar checks
• reject invalid dates early with a clear error response
• harden dailyPath() to throw on invalid date input
• add regression tests for invalid/traversal dates

Why

memory_read previously accepted arbitrary date strings, which could escape the daily directory when joined into a path.

Closes #3

Verification

• npm test
• npm run lint
• npm run build
• bun test test/unit.test.ts -t "memory_read tool|dailyPath"