リファクタリング Phase 3: ツーリング整備（ameba / CI / spec）

[limit7412]

Part of #84（3分割 PR の 3/3）

⚠️ base は refactor/phase-2-design（PR #87）。Phase 1 → 2 → 3 の順でマージしてください。

ツーリングとテストの整備。

• ameba 1.6 を development_dependency に追加し指摘を解消
• コンパイル不能だった spec（存在しないファイルを require）を修正し、プレースホルダ（false.should eq(true)）をモデルのユニットテストに置換
• GitHub Actions CI（format / ameba / spec）を追加
• 死んでいた .travis.yml を削除、ビルド成果物 bin/ を gitignore

検証: crystal spec 11 examples 0 failures / ameba 13 inspected 0 failures。

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request cleans up the project structure by removing Travis CI configuration, adding the linter Ameba to development dependencies, and renaming rescued exception variables from err to error for better clarity. It also introduces comprehensive unit tests for the GitHub and Slack models. Regarding the feedback, it is recommended to provide a fallback (such as error.class.name) when error.message is nil in src/runtime/lambda.cr to avoid returning a null error field and to improve debuggability.

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