docs: Update documentation to reflect starter pack threshold feature
- Add docs/common/starterPackThreshold.md for new module
- Update types.md with WindowUnit, StarterPackThresholdConfig
- Update redis.md with starter pack tracking functions and fixed signatures
- Update metrics.md with 3 new starter pack threshold metrics
- Update accountThreshold.md with corrected function signature (uri param)
- Update accountModeration.md with error handling clarification
- Update main.md with starter pack handler section
- Update developing_checks.md with starter pack threshold section
- Fix accountThreshold config examples (windowDays → window + windowUnit)
- Update README.md with complete env vars and threshold system docs
- Improve rule file examples with comprehensive optional field coverage
- Add proper LINK_SHORTENER pattern to constants.ts
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>