Add automatic triage labeling for new issues

[Copilot]

Newly opened issues were not being marked for initial review. This change adds repository-level automation so every new issue receives a triage label consistently.

• What changed

  • Added a new GitHub Actions workflow at .github/workflows/triage.yml
  • Triggers on issues.opened
  • Applies the triage label to the newly created issue

• Label bootstrap

  • If the triage label does not already exist, the workflow creates it before applying it
  • Keeps the automation self-contained and avoids a manual repository setup dependency

• Workflow behavior

  on:
    issues:
      types:
        - opened

• Implementation details

  • Uses actions/github-script to:
    • check for the triage label
    • create it on first use if needed
    • attach it to the opened issue

Original prompt

issueが立ったらtriageタグをつけてほしい

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