feat: honor --ago cutoff when purging untagged manifests

[balcsida]

• Apply the purge command’s age threshold to manifest cleanup by deriving the cutoff in purgeDanglingManifests and filtering untagged manifests by LastUpdateTime before deletion so only older content is removed.
• Keep annotation support aligned with the new helper signature by passing a nil cutoff when collecting untagged manifests for annotate workflows.
• Document the age-respecting manifest purge behavior and add unit plus smoke coverage to confirm recent untagged images are preserved until they age past the cutoff.

Discovered while finding answer for #515

[FeynmanZhou]

Can we also update the command examples for the flag --ago to tell users about how to use --ago with --untagged together to configure a fine-grained age threshold of the purge policy?

[estebanreyl]

Can we add the examples to the consts at the top of purge.go? Also updating the command info for --ago there as it currently states:

cmd.Flags().StringVar(&purgeParams.ago, "ago", "", "The tags that were last updated before this duration will be deleted, the format is [number]d[string] where the first number represents an amount of days and the string is in a Go duration format (e.g. 2d3h6m selects images older than 2 days, 3 hours and 6 minutes)")

which wouldn't reflect the new changes. I think beyond that the PR is ready.

[balcsida]

Sure thing, I'll make this small update as soon as I can

[FeynmanZhou]

Thanks @balcsida for the 4th PR! @estebanreyl will help review the code.

[Copilot]

Pull Request Overview

This PR implements age-based filtering for untagged manifest purging to honor the --ago cutoff parameter. The change ensures that when using the --untagged flag with --ago, only untagged manifests older than the specified time threshold are deleted, preserving recent content.

Key changes:

• Extended GetUntaggedManifests function to accept an optional age cutoff parameter
• Modified purgeDanglingManifests to calculate and pass the age threshold to manifest filtering
• Updated all call sites to maintain compatibility with the new function signature

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
cmd/repository/image_functions.go	Added age filtering logic to respect --ago cutoff when collecting untagged manifests
cmd/acr/purge.go	Modified to parse age duration and pass cutoff to manifest collection
cmd/acr/annotate.go	Updated to pass nil cutoff for annotation workflows
cmd/acr/purge_test.go	Added unit tests and updated existing tests to use new function signature
scripts/experimental/test-purge-all.sh	Added smoke tests to verify age-based manifest filtering behavior
README.md	Updated documentation to explain age-respecting manifest cleanup

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[Copilot]

The test references lastUpdateTime2DaysAgo variable but this variable is not defined in the visible test context. This will cause a compilation error.

Suggested change

	manifestList := &acr.Manifests{
	Registry: &testLoginURL,
	ImageName: &testRepo,
	ManifestsAttributes: &[]acr.ManifestAttributesBase{{
	LastUpdateTime: &lastUpdateTime,
	lastUpdateTime2DaysAgo := time.Now().Add(-48 * time.Hour)
	manifestList := &acr.Manifests{
	Registry: &testLoginURL,
	ImageName: &testRepo,
	ManifestsAttributes: &[]acr.ManifestAttributesBase{{
	LastUpdateTime: &lastUpdateTime2DaysAgo,

[Copilot]

The test references lastUpdateTime2DaysAgo variable but this variable is not defined in the visible test context. This will cause a compilation error.

[estebanreyl]

Overall LGTM, left some minor comments.