Add artifact delete endpoint and API test suite; drop internal-only docs from tracking

Adds DELETE /api/artifacts/{id}, exposes supported HTTP methods via
/api/capabilities, and adds a tests/api/ suite covering create/list/detail/
patch/delete lifecycle. Also stops tracking _archive_/, planning/, and
opencode.json, which carry home-infrastructure details and local-machine-only
config not meant to ship in this repo.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

artifacts/views.py

@@ -243,6 +243,16 @@ class ArtifactDetailView(APIView):
         )
         return Response(serialize_artifact(artifact, request))
 
+    def delete(self, request, artifact_id):
+        user = resolve_token_user(request)
+        if user is None:
+            return HttpResponseForbidden("Invalid or missing API token")
+
+        artifact = get_object_or_404(Artifact, pk=artifact_id, owner=user)
+        delete_artifact_files(artifact)
+        artifact.delete()
+        return Response(status=204)
+
 
 class CapabilitiesView(APIView):
     """Static capability info. Reachable by a logged-in session or a valid API token,
@@ -259,6 +269,11 @@ class CapabilitiesView(APIView):
                 "raw_supported_for": [Artifact.MARKDOWN, Artifact.HTML_FULL],
                 "source_types": [choice[0] for choice in Artifact.SOURCE_TYPE_CHOICES],
                 "auth": "bearer_token",
+                "methods": {
+                    "/api/artifacts": ["GET", "POST"],
+                    "/api/artifacts/{id}": ["GET", "PATCH", "DELETE"],
+                    "/api/assets": ["POST"],
+                },
             }
         )