fix: advertise MCP history date format

[vycdev2]

Summary

• advertise the existing YYYY-MM-DD constraint in the read-message-history MCP input schema
• cover the published schema with a regression assertion
• document the client-visible schema fix in the changelog

Verification

• npm run build — passed: TypeScript compilation completed
• node --test tests/mcpHistory.test.mjs — passed: 1 test
• npm test — passed: all 109 tests
• git diff --check — passed

Risk

• Low: this only adds JSON Schema metadata matching the date validation already enforced at runtime.

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