Add GitHub Actions to Dependabot configuration

[Copilot]

This PR adds GitHub Actions monitoring to the Dependabot configuration, following the existing pattern for npm package updates.

Changes

Added a new github-actions package ecosystem entry to .github/dependabot.yml that:

• Monitors GitHub Actions versions used in workflows (e.g., actions/checkout@v4, cloudflare/wrangler-action@v3)
• Uses the same monthly update schedule as the existing npm configuration
• Follows the same configuration pattern for consistency

Benefits

Dependabot will now automatically create pull requests when newer versions of GitHub Actions are available, helping to:

• Keep workflows up-to-date with the latest security patches
• Benefit from new features and improvements in GitHub Actions
• Maintain consistency with the existing dependency update strategy

Closes #[issue_number]

Original prompt

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This section details on the original issue you should resolve

<issue_title>setup github actions dependabot</issue_title>
<issue_description></issue_description>

<agent_instructions>既存の dependabot の設定の真似をしろ</agent_instructions>

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

Fixes #53

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