Add Gentoo distfiles proxy support with cache exclude feature

[ganto]

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[Copilot]

Pull request overview

This PR adds Gentoo distfiles proxying support to pkgproxy by enabling a “cache everything” mode via a suffixes: ["*"] wildcard, plus a per-repository cache exclude list to prevent caching selected metadata files (e.g., layout.conf). It also introduces a Gentoo end-to-end test and updates user-facing documentation/snippets to reflect the new distro support.

Changes:

• Add exclude support to repository/cache configuration and apply it in cache candidacy decisions (with "*" wildcard semantics).
• Add Gentoo repository configuration and landing-page/README configuration snippets.
• Add Gentoo e2e coverage (stage3 container + emerge --fetchonly) and supporting test script.

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
test/e2e/test-gentoo.sh	New Gentoo e2e script that bootstraps Portage, fetches via proxy, and exercises “not cached” path for layout.conf.
test/e2e/e2e_test.go	Adds Gentoo e2e test and a helper to assert files are not cached.
README.md	Documents Gentoo GENTOO_MIRRORS configuration and updates supported DISTRO list.
pkg/pkgproxy/repository.go	Adds Exclude to repo config and logs a warning for redundant suffix config when "*" is present.
pkg/pkgproxy/repository_test.go	Unit tests validating the new wildcard redundancy warning behavior.
pkg/pkgproxy/proxy.go	Wires repository Exclude config into cache construction.
pkg/pkgproxy/landing.go	Adds Gentoo snippet for the landing page repository list.
pkg/cache/cache.go	Implements exclude-first logic and "*" wildcard cache candidacy.
pkg/cache/cache_test.go	Adds unit tests for exclude/wildcard behavior in IsCacheCandidate.
openspec/changes/gentoo-distfiles-proxy/tasks.md	Implementation task checklist for the change.
openspec/changes/gentoo-distfiles-proxy/specs/gentoo-distfiles/spec.md	Spec requirements for Gentoo distfiles support and non-caching of layout.conf.
openspec/changes/gentoo-distfiles-proxy/specs/e2e-multi-distro/spec.md	Spec requirements for Gentoo e2e coverage.
openspec/changes/gentoo-distfiles-proxy/specs/cache-exclude/spec.md	Spec requirements for wildcard caching and exclude semantics.
openspec/changes/gentoo-distfiles-proxy/proposal.md	Proposal describing motivation and intended changes.
openspec/changes/gentoo-distfiles-proxy/design.md	Design notes and trade-offs for wildcard+exclude and Gentoo e2e approach.
openspec/changes/gentoo-distfiles-proxy/.openspec.yaml	Openspec metadata for the change set.
Makefile	Adds gentoo mapping to make e2e DISTRO=gentoo.
configs/pkgproxy.yaml	Adds gentoo repository entry (wildcard suffix + exclude list + mirrors).
CHANGELOG.md	Documents the added Gentoo support and cache exclude feature under Unreleased.

Comments suppressed due to low confidence (1)

test/e2e/e2e_test.go:180

• In assertCachedFiles, filepath.Walk's returned error is ignored and walk errors are swallowed in the callback. This can make failures look like 'no cached files' rather than surfacing the underlying filesystem issue. Capture the error from filepath.Walk and fail the test when it's non-nil (and consider propagating callback errors).

func assertCachedFiles(t *testing.T, cacheDir string, repoPrefix string, suffix string) {
	t.Helper()
	var matches []string
	filepath.Walk(filepath.Join(cacheDir, repoPrefix), func(path string, info os.FileInfo, err error) error {
		if err != nil {
			return nil
		}
		if !info.IsDir() && strings.HasSuffix(path, suffix) {
			matches = append(matches, path)
		}
		return nil
	})
	assert.NotEmpty(t, matches, "expected %s files under %s/%s", suffix, cacheDir, repoPrefix)

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