pinning actions, adding dependabot

[Ryiguchi]

No description provided.

[Copilot]

Pull request overview

This PR improves supply-chain security and maintenance of this repository’s GitHub Actions automation by pinning third-party actions to immutable commit SHAs and introducing Dependabot automation for GitHub Actions updates.

Changes:

• Pinned GitHub Actions used across workflows and composite actions to specific commit SHAs (with version comments).
• Standardized action references in test, publish, validation, and deployment automation.
• Added a .github/dependabot.yml configuration to enable weekly GitHub Actions update PRs (grouped for minor/patch updates).

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
workflows/test-in-parallel.yml	Pins all referenced actions (checkout, setup-node, bun/pnpm setup, 1Password, notifications) to SHAs.
workflows/expo-publish-pr.yml	Pins checkout, bun setup, caching, expo actions, and discord notification action to SHAs.
actions/healthcheck/action.yml	Pins health check and discord notification actions to SHAs.
actions/docker-build/action.yml	Pins Docker login/buildx/build-push actions to SHAs.
actions/caprover-setup/action.yml	Pins bun setup and discord notification actions to SHAs.
actions/caprover-deploy/action.yml	Pins bun setup action to an SHA.
.github/workflows/validate-yaml.yml	Pins checkout action to an SHA for YAML/action linting workflow.
.github/workflows/validate-plugins.yml	Pins checkout and bun setup actions to SHAs for plugin validation workflow.
.github/dependabot.yml	Adds Dependabot configuration to keep GitHub Actions dependencies updated.

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