ci: Add concurrency control to cancel stale PR runs

[Copilot]

When multiple commits are pushed to a PR in quick succession, redundant CI runs pile up. This adds concurrency groups to cancel in-progress workflow runs when a newer commit triggers a new run on the same ref.

Changes

• ci.yml: Adds concurrency group scoped to workflow + ref. Uses a conditional cancel-in-progress — true for PRs, false for master to avoid interrupting active deployments.
• actionlint.yml: Adds concurrency group with unconditional cancel-in-progress: true (no deployment concern).

concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}
  cancel-in-progress: ${{ github.ref != 'refs/heads/master' }}

Original prompt

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This section details on the original issue you should resolve

<issue_title>CI concurrency</issue_title>
<issue_description>一つのPRに対して同時に走るworkflowは一つで良い．それ以前のコミットによってトリガーされた実行中のCIはキャンセルするようにしてほしい．</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes CI concurrency #776

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[SnO2WMaN]

@copilot

Update commit message

[Copilot]

@copilot

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