ci: golangci-lint の GitHub Actions を追加 (#373)

[taminororo]

Summary

CodeRabbit は golangci-lint を root 起点で ./... 実行し、Go モジュールが api/ サブディレクトリにある本リポでは原理的に動かせない（#373）。回数無制限・CodeRabbit のクレジットに非依存な GitHub Actions で golangci-lint を working-directory: api 実行に置き換える。

Refs #373

ワークフロー

- uses: golangci/golangci-lint-action@v7
  with:
    version: v2.12
    working-directory: api      # ← CodeRabbit に無い、api/ で実行する指定
    only-new-issues: true       # ← 既存負債(約74件)を無視し、PR新規分だけ検査

• working-directory: api: go.mod のある api/ で実行。CodeRabbit の directory prefix . does not contain main module を構造的に回避。
• only-new-issues: true: 既存の lint 負債（ineffassign 約70・gosec 2・unused 2）で全PRが赤くならないよう、PR の差分で新規に増えた指摘だけを落とす（--new-from-patch）。
• action は golangci-lint v2 対応の v7、version: v2.12（CodeRabbit と同じ系統）。

実地検証済み

PR #372 のブランチ上でこのワークフローを動かし、成功を確認済み：

Running golangci-lint-2.12.2 run --new-from-patch=... --path-prefix=api
  in [/home/runner/work/SeeFT/SeeFT/api]   ← api/ で実行されている
"0 issues." / "golangci-lint found no issues" → 45秒で pass（緑）

補足

• このPRは .github/workflows/ のみの変更で api/** を含まないため、path filter によりこのPR自体では lint は起動しない（仕様）。develop マージ後、api/ を変更する以後の全PRで自動起動する。
• CodeRabbit の golangci-lint は chore: CodeRabbit に golangci-lint の config_file を明示 (#373) #374（config_file）で改善が見込めるため無効化せず併存（クレジットがある時のレビュー補助として残す）。CI が確実な lint の担保、CodeRabbit は best-effort、という二段構え。
• ログに Node 20 deprecation 警告が出るが非ブロッキング。将来 checkout@v5 / action@v8+ に上げれば解消。

Test plan

• PR fix: errcheck（e.Start / c.JSON）を解消 (#316) #372 ブランチ上で golangci-lint が api/ で動作し pass することを確認済み
• develop マージ後、次に api/ を変更する PR で自動起動・pass することを確認

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 0e1bfb24-bc5d-4113-83e7-5ac8dd64fd6f

📥 Commits

Reviewing files that changed from the base of the PR and between 1afd465 and 5719d27.

📒 Files selected for processing (1)

• .github/workflows/lint.yml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/golangci-lint-actions

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