リファクタリング Phase 1: イディオム・スタイル最新化

[limit7412]

Part of #84（3分割 PR の 1/3）

最新の Crystal（1.20系）らしいイディオム・スタイルへ更新。挙動は不変。

• shard.yml: crystal 要件を >= 1.20.0 に
• lambda.cr: puts `echo '...'` のサブシェルログ（インジェクション/クォート破壊リスク）を stdlib Log に置換。handler のブロックパラメータを明示
• github/repository.cr: 数値比較を HTTP::Status 述語（server_error? / unauthorized? / client_error?）へ。typo (retrun/faild) 修正
• github/models.cr: property→getter、冗長な emit_null: false 削除、[...].includes?→定数配列+in?、comment_url を presence || 化
• error/usecase.cr: err.backtrace を backtrace?.try で安全に
• notify/usecase.cr: size != 0→empty? 否定
• slack/models.cr: property→getter

検証: crystal tool format --check 差分なし / crystal build 成功。

後続: Phase 2 (#87) → Phase 3 (#88) がこの上にスタックされます。

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request refactors the codebase to use modern Crystal idioms, such as changing properties to getters, utilizing semantic HTTP status checks, and simplifying conditional logic. It also replaces shell execution in print_log with the standard Log library. The review feedback suggests further performance optimizations: using Tuple instead of Array for static constants (MENTION_REASONS and UPDATE_TYPES) to avoid heap allocations, and refactoring print_log to use index-based string slicing instead of each_char.each_slice to prevent intermediate array allocations.

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