fix: ignore local markdown translation artifacts

[zzgosh]

Summary

• allow ignored Markdown Translate outputs (*_<language>_mdt.md) to remain in resource directories without changing local tree hashes
• fail locally when matching translation outputs are not ignored, and keep other ignored untracked artifacts fail-closed
• document the refined prepare/verify behavior and update agent-collaboration guidance

Validation

• pnpm agents prepare
• pnpm agents verify
• pnpm agents:test
• pnpm exec tsc --noEmit
• git diff --check