Fix: CI actions are unpinned and actions/cache is a major version behind (Auto-Generated)

[Ennyblack]

Closes #174

This pull request was generated automatically and scoped strictly to issue #174.

Changes

Pin GitHub action versions in ci.yml to 40-character commit SHAs, bump actions/cache to v4 (pinned to a v4 commit), and add dependabot.yml covering github-actions and cargo ecosystems weekly.

Verification

⚠️ Not verified locally (no build system detected, or the required toolchain isn't installed on the worker). GitHub CI is the source of truth — please check the CI status on this PR before merging.

Linked with Closes #174 so the Drips Wave bot resolves the issue on merge.

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[mergekeeper]

MergeKeeper review

Scope: in scope for linked issue #174.
Verdict: clean

The pull request correctly pins all GitHub actions to commit SHAs, upgrades actions/cache to v4, and adds a properly configured dependabot.yml covering github-actions and cargo ecosystems.

Reviewed commit: cb5ece8ef71e7bdfed77cbef925d39343e3e79f6.
CI and merge eligibility are checked separately.

[mergekeeper]

Needs review

Linked to #174, but the diff does not match the issue scope.

The pull request is missing the required .github/dependabot.yml configuration file and actions/cache upgrades specified in the issue.

Reviewed commit: c1495c7e3d17486b321253026d474d8bef9e3cb2.