ci: add Dependabot for cargo and GitHub Actions

[don-petry]

Why?

Dependencies with known vulnerabilities or outdated actions can introduce security risks and subtle breakage. Dependabot automates the tedious work of monitoring and proposing updates, keeping the supply chain current without manual effort.

Summary

• Add .github/dependabot.yml to enable automated dependency updates
• Configures weekly Cargo dependency updates (prefixed deps)
• Configures weekly GitHub Actions updates (prefixed ci)

Addresses item 5 in #11.

Test plan

• Verify Dependabot opens PRs on the configured schedule
• Confirm PR prefixes match the configured commit-message settings

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a Dependabot configuration to the repository to automate weekly dependency update PRs for both Rust (Cargo) dependencies and GitHub Actions workflow dependencies, aligning with CI/quality improvements in issue #11.

Changes:

• Add .github/dependabot.yml enabling weekly updates for the cargo ecosystem at repo root, with commit message prefix deps.
• Add weekly updates for the github-actions ecosystem at repo root, with commit message prefix ci.

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