Add dependabot config for github-actions ecosystem

[nanaf6203-bit]

Context

.github/dependabot.yml configures weekly npm updates with groups, but does not include a package-ecosystem: github-actions entry, so workflow actions (actions/checkout, actions/setup-node, actions/upload-artifact, ...) are never proposed for bumps.

Problem

Action versions drift and known security issues in popular actions are not surfaced by Dependabot, while npm deps already benefit from automated PRs.

Proposed approach

Add a github-actions ecosystem entry (weekly schedule, labels matching the npm config, optional grouping), keep the existing npm configuration intact.

Acceptance criteria

• dependabot.yml includes package-ecosystem: github-actions
• Dependabot opens PRs for action version bumps
• npm ecosystem config unchanged and still grouped

[Mitch5000]

@Mitch5000 has applied to work on this issue as part of the Stellar Wave Program's 9th wave.

I would like to work on this please.

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[khalidNiass]

@khalidNiass has applied to work on this issue as part of the Stellar Wave Program's 9th wave.

I can handle this. I have GitHub Actions and CI experience and can add the Dependabot config without affecting the existing npm setup.

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[eulami]

@eulami has applied to work on this issue as part of the Stellar Wave Program's 9th wave.

Hi. Kindly assign me to fix this issue

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This issue has been completed by @eulami as part of the Stellar Wave Program's 9th Wave 🥳

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