docs(specification): add deleted_at field to agents project delete JSON/YAML output

Updated docs/specification.md to document the correct JSON/YAML output structure
for the project delete command, replacing the legacy deletion_summary object with
the deleted, success, and deleted_at fields that match the actual implementation
introduced in PR #6639. Added BDD feature file and step definitions to validate
the output format.

ISSUES CLOSED: #7872

docs/specification.md

@@ -3605,21 +3605,9 @@ Delete a project and unlink all associated resources. The resources themselves r
       "status": "ok",
       "exit_code": 0,
       "data": {
-        "deletion_summary": {
-          "project": "local/docs",
-          "resources_unlinked": 1,
-          "data_dir": "/repos/docs/.cleveragents"
-        },
-        "index_cleanup": {
-          "text_index": "cleared",
-          "vectors": "240 removed",
-          "graph_triples": "none",
-          "storage_freed_mb": 12
-        },
-        "backups": {
-          "snapshot": "/backups/local-docs-2026-02-08.tgz",
-          "retention_days": 7
-        }
+        "deleted": "local/docs",
+        "success": true,
+        "deleted_at": "2026-04-10T19:45:43.123456+00:00"
       },
       "timing": { "duration_ms": 530 },
       "messages": [{ "level": "ok", "text": "Project deleted" }]
@@ -3633,18 +3621,9 @@ Delete a project and unlink all associated resources. The resources themselves r
     status: ok
     exit_code: 0
     data:
-      deletion_summary:
-        project: local/docs
-        resources_unlinked: 1
-        data_dir: /repos/docs/.cleveragents
-      index_cleanup:
-        text_index: cleared
-        vectors: 240 removed
-        graph_triples: none
-        storage_freed_mb: 12
-      backups:
-        snapshot: /backups/local-docs-2026-02-08.tgz
-        retention_days: 7
+      deleted: local/docs
+      success: true
+      deleted_at: '2026-04-10T19:45:43.123456+00:00'
     timing:
       duration_ms: 530
     messages: