Add builtin functions for OCI artifacts discovery

acceptance/examples/image_referrers.rego

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+package referrers
+
+import rego.v1
+
+# METADATA
+# custom:
+#   short_name: count
+deny contains result if {
+	refs := ec.oci.image_referrers(input.image.ref)
+	count(refs) != 2
+	result := {
+		"code": "referrers.count",
+		"msg": sprintf("Expected 2 referrers, got %d: %v", [count(refs), refs]),
+	}
+}
+
+# METADATA
+# custom:
+#   short_name: format
+deny contains result if {
+	descriptors := ec.oci.image_referrers(input.image.ref)
+	not all_descriptors_valid_format(descriptors)
+	result := {
+		"code": "referrers.format",
+		"msg": sprintf("Invalid referrer descriptor format in: %v", [descriptors]),
+	}
+}
+
+# METADATA
+# custom:
+#   short_name: content_types
+deny contains result if {
+	descriptors := ec.oci.image_referrers(input.image.ref)
+	not has_expected_artifact_types(descriptors)
+	result := {
+		"code": "referrers.content_types",
+		"msg": sprintf("Expected one signature and one attestation artifact type in referrers: %v", [descriptors]),
+	}
+}
+
+all_descriptors_valid_format(descriptors) if {
+	every descriptor in descriptors {
+		# Each descriptor should have required fields
+		descriptor.digest != ""
+		descriptor.mediaType != ""
+		descriptor.size >= 0
+		descriptor.artifactType != ""
+		descriptor.ref != ""
+
+		# Digest should be a digest-only format: sha256:<hex>
+		startswith(descriptor.digest, "sha256:")
+		not contains(descriptor.digest, "@")
+
+		# Ref should be a full OCI reference with digest format: registry/repo@sha256:<hex>
+		contains(descriptor.ref, "@")
+		contains(descriptor.ref, "sha256:")
+		# Split by @ and verify format
+		parts := split(descriptor.ref, "@")
+		count(parts) == 2
+		# Verify digest format matches
+		parts[1] == descriptor.digest
+	}
+}
+
+has_expected_artifact_types(descriptors) if {
+	# Check that we have one signature artifact directly from descriptors
+	signature_artifacts := [d |
+		some d in descriptors
+		d.artifactType == "application/vnd.dev.cosign.simplesigning.v1+json"
+	]
+	count(signature_artifacts) == 1
+
+	# Check that we have one attestation artifact directly from descriptors
+	attestation_artifacts := [d |
+		some d in descriptors
+		d.artifactType == "application/vnd.dsse.envelope.v1+json"
+	]
+	count(attestation_artifacts) == 1
+}

acceptance/examples/image_tag_refs.rego

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+package tag_refs
+
+import rego.v1
+
+# METADATA
+# custom:
+#   short_name: count
+deny contains result if {
+	refs := ec.oci.image_tag_refs(input.image.ref)
+	count(refs) != 2
+	result := {
+		"code": "tag_refs.count",
+		"msg": sprintf("Expected 2 tag-based artifact references, got %d: %v", [count(refs), refs]),
+	}
+}
+
+# METADATA
+# custom:
+#   short_name: format
+deny contains result if {
+	refs := ec.oci.image_tag_refs(input.image.ref)
+	not all_refs_valid_format(refs)
+	result := {
+		"code": "tag_refs.format",
+		"msg": sprintf("Invalid tag reference format in: %v", [refs]),
+	}
+}
+
+# METADATA
+# custom:
+#   short_name: sig_count
+deny contains result if {
+	refs := ec.oci.image_tag_refs(input.image.ref)
+	sig_count := count([ref | some ref in refs; contains(ref, ".sig")])
+	sig_count != 1
+	result := {
+		"code": "tag_refs.sig_count",
+		"msg": sprintf("Expected 1 .sig reference, got %d", [sig_count]),
+	}
+}
+
+# METADATA
+# custom:
+#   short_name: att_count
+deny contains result if {
+	refs := ec.oci.image_tag_refs(input.image.ref)
+	att_count := count([ref | some ref in refs; contains(ref, ".att")])
+	att_count != 1
+	result := {
+		"code": "tag_refs.att_count",
+		"msg": sprintf("Expected 1 .att reference, got %d", [att_count]),
+	}
+}
+
+all_refs_valid_format(refs) if {
+	every ref in refs {
+		# Each ref should be a valid OCI reference with tag format: registry/repo:sha256-<hex>.<suffix>
+		contains(ref, ":")
+		contains(ref, "sha256-")
+		# Split by : and get the last part (the tag)
+		parts := split(ref, ":")
+		tag_part := parts[count(parts) - 1]
+		# Tag should start with sha256- and end with .sig or .att
+		startswith(tag_part, "sha256-")
+		valid_suffix(tag_part)
+	}
+}
+
+valid_suffix(tag) if {
+	endswith(tag, ".sig")
+}
+
+valid_suffix(tag) if {
+	endswith(tag, ".att")
+}