feat: add action update cooldown

[qartik]

Summary

• add an optional --cooldown-days flag to gate upgrades on tag age
• resolve effective tag publish times from GitHub git refs for annotated and lightweight tags
• cover cooldown selection, unchanged reporting, and CLI parsing with tests

Testing

• go test ./...

Fixes #1

Drive by

• Added a new CI workflow configuration in .github/workflows/ci.yml to ensure tests are run on pull requests and pushes to main.

[Copilot]

Pull request overview

Adds an optional “update cooldown” feature to prevent upgrading GitHub Action tags that are newer than a specified age, improving supply-chain hygiene by avoiding freshly-published tags.

Changes:

• Introduces --cooldown-days CLI flag and threads its value through report planning and GitHub tag resolution.
• Enhances GitHub client logic to determine tag “publish time” via git refs (handling annotated vs lightweight tags) and caches tag timestamps.
• Adds tests covering cooldown behavior, unchanged reporting reasons, and CLI parsing.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
README.md	Documents --cooldown-days usage and how it affects eligibility of tags.
internal/github/client.go	Implements cooldown filtering by resolving tag timestamps from GitHub git refs and caching results.
internal/github/client_test.go	Adds unit tests for cooldown selection, timestamp source (annotated/lightweight), and caching.
cmd/actupdate/main.go	Adds flag parsing and passes cooldown duration into planning/resolution.
cmd/actupdate/main_test.go	Adds CLI parsing tests and end-to-end-ish run tests for cooldown scenarios.

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[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.