Refactor external_resources from class attributes to shared slot

Convert external_resources from duplicated class attribute definitions
to a shared LinkML slot following best practices.

Changes:
- Add external_resources slot to D4D_Base_import.yaml with dcterms:references URI
- Update ExternalResource class to use slots + slot_usage (range: string)
- Update Dataset class to use slots + slot_usage (range: ExternalResource)
- Remove duplicate attribute definition from Dataset

Benefits:
- Single source of truth for external_resources semantics
- Follows LinkML best practices for slot reuse
- Easier maintenance with centralized slot_uri and properties
- No functional changes - maintains backward compatibility

The recursive structure still works correctly:
- Dataset.external_resources → list of ExternalResource objects
- ExternalResource.external_resources → list of URL strings

All tests pass and existing D4D YAML files validate successfully.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: realmarcin

examples/output/CleaningStrategy-minimal.json

@@ -1,5 +1,4 @@
 {
   "id": "data_sheets_schema:123",
-  "description": "[]",
   "@type": "CleaningStrategy"
 }

examples/output/CleaningStrategy-minimal.yaml

@@ -1,2 +1 @@
 id: data_sheets_schema:123
-description: '[]'

examples/output/CleaningStrategy-valid.json

@@ -1,5 +1,4 @@
 {
   "id": "data_sheets_schema:123",
-  "description": "[]",
   "@type": "CleaningStrategy"
 }

examples/output/CleaningStrategy-valid.yaml

@@ -1,2 +1 @@
 id: data_sheets_schema:123
-description: '[]'

examples/output/CollectionConsent-minimal.json

@@ -1,5 +1,4 @@
 {
   "id": "data_sheets_schema:123",
-  "description": "[]",
   "@type": "CollectionConsent"
 }

examples/output/CollectionConsent-minimal.yaml

@@ -1,2 +1 @@
 id: data_sheets_schema:123
-description: '[]'

examples/output/CollectionConsent-valid.json

@@ -1,5 +1,4 @@
 {
   "id": "data_sheets_schema:123",
-  "description": "[]",
   "@type": "CollectionConsent"
 }

examples/output/CollectionConsent-valid.yaml

@@ -1,2 +1 @@
 id: data_sheets_schema:123
-description: '[]'

examples/output/CollectionMechanism-minimal.json

@@ -1,5 +1,4 @@
 {
   "id": "data_sheets_schema:123",
-  "description": "[]",
   "@type": "CollectionMechanism"
 }

examples/output/CollectionMechanism-minimal.yaml

@@ -1,2 +1 @@
 id: data_sheets_schema:123
-description: '[]'