Pin GitHub Actions to full-length commit SHAs (#99)

## Summary

This PR pins GitHub Actions to full-length commit SHAs for improved
security and reproducibility and adds a 7 day cooldown to Dependabot
configuration for GitHub Actions. This work is described in more detail
at https://aka.ms/action-pinning.

## Why?

Pinning actions to commit SHAs prevents supply-chain attacks where a tag
could be moved to point to malicious code. This is a recommended
security best practice per the [GitHub Actions security hardening
guide](https://docs.github.com/en/actions/security-for-github-actions/security-guides/security-hardening-for-github-actions#using-third-party-actions).

This change mitigates the risk of tag retargeting to malicious code as
seen in incidents like the [tj-actions/changed-files action
compromise](https://www.stepsecurity.io/blog/harden-runner-detection-tj-actions-changed-files-action-is-compromised)
or [codfish/semantic-release-action
compromise](https://www.stepsecurity.io/blog/supply-chain-compromise-codfish-semantic-release-action)
and improves the integrity and reproducibility of the CI/CD pipeline.

## What changed?

**Action pinning:** Third-party action references in
`.github/workflows/` that used mutable tag-based references (e.g.,
`actions/checkout@v4`) have been updated to full-length commit SHAs with
a version comment (e.g., `actions/checkout@<sha> # v4`) using the
[pinact](https://github.com/suzuki-shunsuke/pinact) tool. References
that were already pinned to a SHA, or that used immutable release tags,
were left unchanged.

**Dependabot configuration:** `.github/dependabot.yml` has been updated
to ensure a `github-actions` package-ecosystem section is present with a
`cooldown` configuration (`default-days: 7`). If the file did not exist,
it was created. If a `github-actions` section already existed, only the
`cooldown` block was added or its `default-days` value was increased to
7 if it was lower. The 7-day cooldown provides a window for the
community to detect and report compromised releases before they are
automatically proposed as updates, reducing exposure to supply-chain
attacks via newly published malicious versions.

## Is this safe to merge?

Yes. The pinned SHAs correspond to the same commits that the existing
tags pointed to. No behavioral changes in action execution are
introduced. You can verify the pinned SHA value using the GitHub REST
API (e.g., the commit hash for `actions/checkout@v7` can be found in the
`sha` property in the JSON response for `GET
https://api.github.com/repos/actions/checkout/commits/v7`).

## Additional Information

For more information, please see https://aka.ms/action-pinning